Lessor’s responsibilities

Get legal/surveyor expertise, leaseholds can be complicated and individual to your lease/circumstances.

If the DPC is bridged I would say there is no question that that is a matter you, as the Landlord, need to rectify at your expense.
There are actions you could take to mitigate the build up of excess condensate; improved insulation, better ventilation and more effective heating . However if the tenant does not ventilate and heat the room in question that would be down to him.

"The specialist has also stated that ‘The damp course along the front and side of the property is bridged by high ground levels’ and that ‘This could be contributing to the effects of condensation.’
They recommend that the condensation is first controlled and then the inadequate bridging addressed. The specialist’s quote to rectify the above issue is for a not inconsiderable amount."
This bloke's an idiot!
1. Don't employ him or his firm again.
2. Get the DPC bridging sorted ASAP irrespective of whether it proves to be relevant to your problem or not.
3. Then sort out the remaining damp problem if it still exists.

If a damp course is bridged then I would have thought that this is a Freeholder responsibility as effects the external part of a building. I presume "high ground levels" refers to flower beds/buildup of earth etc. If you all jointly own the Freehold then any works should be shared between all of you.

Lack of ventilation: You dont say whether this is a purpose-built block or flats in a conversion. Or what floor your flat is on. Or where the damp is showing.

If ground floor flat are there airbricks below the damp proof course level? Are they covered up by the earth and so blocked? Removing the building up of earth and clearing the holes will allow ventilation under the property.https://www.travisperkins.co.uk/product/building-materials/bricks-and-blocks/brickwork-ventilation/c/1500038/

If a conversion it may well be whoever converted the flats plastered over existing airbricks. Are any visible from outside? If so then just need opening up from the inside and a decorative grill placed over. If nothing can be seen then you can easily install a passive air vent (kit available cheaply from Travis Perkins or any other good builders merchants even Amazon https://www.amazon.co.uk/Cowled-Vent-Set-Passive-Brown/dp/B07PGVT11J/ref=sr_1_13?adgrpid=54801848122&dchild=1&gclid=EAIaIQobChMI28S4mtHS7gIVGO7tCh2IwAsNEAMYAiAAEgIqjfD_BwE&hvadid=259039010081&hvdev=c&hvlocphy=9045584&hvnetw=g&hvqmt=b&hvrand=2735344710514847333&hvtargid=kwd-297944412421&hydadcr=20969_1721632&keywords=passive+vent&qid=1612524770&sr=8-13&tag=googhydr-21).

Where is the damp in the bedroom? Only appears in the autumn/winter/early spring? Is the window double glazed? With or without a trickle vent? Can be locked open 1cm?

Damp/condensation most likely to be lifestyle of the tenants eg not ventilating the room (opening the window); not having the trickle vent open at night; not leaving the window open a tiny bit at night; not wiping condensation from the window in the morning; having wardrobes/chests of drawers tight up against an outside wall; not having the heating on adequately.

Trouble is you cannot tell a tenant how to live 🙁

"Trouble is you cannot tell a tenant how to live"
I have a large upmarket four bed detached that I have rented to high income tenants for about ten years now.
I had a huge mould problem in one bed room that was occupied by a senior manager.
The tenant was continually ill with, amongst other things, chest infections.
Nobody ever complained about the extensive black mould.
As usual, at tenant change, I thoroughly cleaned and decorated the room and I gave the tenant his full deposit back.
The outgoing tenant returned to pick up a couple of things and made it very clear that he thought that cleaning and decorating the room was completely unnecessary.
I had no damp or mould in the property before this particular tenant moved in, or at any time since.
When I washed the curtains, they were so rotten that they disintegrated.............

Slightly off-topic, but I'm surprised by the absence of discussion here (P118) about the Cladding and Fire Safety scandal which is currently blighting the leasehold community.
Innocent LH are subject to exorbitant demands (and bankruptcy) to fix historic defects in buildings they don't own - and had no say in the build process or (lack of) regulation control.
Service charges of up to £150,000 (per flat) are now making news.
Building safety fund £1.6b when most estimates are ten times that.
Waking Watch costs of £10k to £20k per month (per building) added to the service charge - for as long as it takes to get the remediation works completed and signed off.
Flats valued at NIL - so you can't sell or remortgage either
Oh yeah, and insurance costs massively increasing.

Reply to the comment left by Paul Shears at 05/02/2021 - 11:20
Thanks for the advice Paul. I assume from your comments that as the main issue here the DPC bridging should be addressed before anything else which could well be attributed to the bridging issue.

Reply to the comment left by Judith Wordsworth at 05/02/2021 - 11:35
Thanks for the advice Judith. I shall certainly investigate the points you raise, although as you say, it is difficult to tell a tenant how to live.The freehold on my purpose-built ground floor flat is not owned by myself. I may approach the Lessor(s) for financial assistance and see how I go but I feel it could be difficult without paperwork clearly listing their responsibilities.

Reply to the comment left by Pauly at 06/02/2021 - 13:07Yes absolutely. Irrespective of whether it is actually the total problem or not, this will be doing the structure no good at all. Also once you have got the DPC bridging sorted, it will take a while (Possibly a couple of months) before the wall dies out fully and you can eliminate it from any remaining issue. You need to get a gap of several inches between the structure and the land and that "trench" needs to be well drained. This may well amount to low skilled labouring work as I have encountered several times in the past. I was younger then and I did everything myself. My costs for the work were too small to remember now., but amounted to no more than a few pounds on both properties.

Reply to the comment left by Paul Shears at 06/02/2021 - 17:17
Thanks for that info Paul. It is really useful and I feel it could save me some money. I will certainly get a few more quotes and go with the one which seems most reasonable and most honest with respect to the DPC bridging (if indeed they raise this as an issue at all) and any required work. The leasehold flat is on the ground floor and is one of a purpose-built block of 4-2 up and 2 down. Do you think that either the the lessor or any of the other 3 owners should assist financially with respect to the DPC bridging? As somebody who knows little about building I have no idea whether the DPC which serves my flat would also serve any of the other 3....Any advice appreciated.