New management company appear out of thin air demanding thousands?

Your first action should be to splash out £3 on a copy of the freehold title, available over the internet from https://www.gov.uk/search-property-information-land-registry

I would be seriously concerned about this. It suggests the Council sold the freehold interest without first offering it to the leaseholders. Also this Man Co has not served a section 3 LTA notice of change of lessor landlord identity.
A search on Companies House register of this company for info on who are directors.
You need to check it’s bank account details too to fulfil Money Laundering regulations. It’s not unknown for service charge not to be paid to the lessor landlord not held in section 42 trust and treated unlawfully as company money.

You can enquire with the so called managing agents as to who the landlord/Head Lessor is using the prescribed form in this link https://www.lease-advice.org/template-document/request-disclosure-landlords-identity-section-1-landlord-tenant-act-1985/
You will also need a good read of your Lease to see what your obligations are for paying service charges and ground rent. Arrears in ground over 6 years cannot be recovered. Any service charges must be reasonable Visit the Leasehold Advisory Service for in depth information. (From a fellow long suffering leaseholder)

Maybe your solicitor's searches were not so thorough after all? They would have known to do a land registry search and I'm somewhat surprised they allowed you to proceed without knowing anything or advising you about a missing freeholder and how freeholder responsibilities were to be carried out and by whom? Looks like the Council has just woken up, but that's not unusual.
My suggestion would be to pay the ground rent asap - likely a small amount anyway that perhaps you can afford to lose if further info comes to light - but just so you cannot be exposed to possible forfeiture of the lease. State you are paying without prejudice to further verification of liability to this freeholder.
Service charges etc can maybe wait until you can verify the credentials of the freeholder/, management co but at £1800 per flat for 6 years its not a huge amount, but hard to justify for having apparently done nothing. Its possible the block was on their block insurance (if they don't self insure) and they are trying to recover premiums. I would query whether you can cross charge for sums expended - like insurance - if there's any double counting.
If not already, you should probably get organised as a group of six so you can stand collectively. Not everyone will have the same appetite to challenge or clarify.

Reply to the comment left by Freda Blogs at 08/11/2023 - 12:09
Hi, thanks for comment. Just for clarity, The sale was delayed due to sols doing thorough searches of info on land registry. The stated management company did not have any involvement and council replied they were not freeholder but are claiming to be now. Also the £1800 is for current year only.

1. If 1800 includes GR that does not sound 'off the plot' for a service charge particularly when a management company is involved.
2. As a freeholder, you generally fill in a form called LPE (leasehold property enquires) before any sale.
There are important forms such as assignment which need to go to the freeholder
Buyers solicitor asks for it.
Sol should have had alarm bells if they could not find anyone to fill in form.
It talks about GR, SC, s20 etc.
I would have a word with your solicitor.
On the plus side you appear not to have paid SC or GR since 2015 and they do not appear to be asking for back payments.
There are strict time limits.

I had a similar issue about 4 years ago. We set up an RTM Co which beat the bas****s. We still have to pay the ground rent annually @ £25, but this is fixed in the lease with no uplift for 999 years. It will eventually cost more for these leaches to collect and administer than it's worth. (I insist on ground rent demands by post to annoy them and I pay by cheque which costs them to pay in - yes, it costs me a quid to post, but I get more than a quid back in satisfaction).

The only costs associated with the RTM Co are the initial formation costs which were around £125. Accounts need to be submitted to Companies House each year so you may have a small accountancy bill if none of you want to take this on.

If any work needs doing, we all chip in our share to the RTM Co.

Hope this helps.

Reply to the comment left by John Hayes at 08/11/2023 - 13:48
Thank you. That is very helpful

Where I was not been able to identify a landlord on a leasehold property I went on to purchase, I took out absent landlord insurance to cover such eventualities. It costs less than £50 typically. Surprised your lawyers did not advise you/ insist on this. I would be talking to them.

Didn’t the stated man. co state why they have no involvement. Are they named as a party to the lease? If so and no longer operating suggests man. Co. Insolvent and solicitors should definitely picked this up and advised you not to proceed. Service charge is only payable to the party named as lessor or landlord in your lease. If it’s this Man. Co. no longer operating then no sc or gr can be paid. The lease cannot be sold on. You’ll need to apply to Treasury Solicitor to have man co reinstated.