Remove my perfectly good UPVC windows and doors and make me pay for the privilege?

It is most likely that doors and windows are yours so the management company cannot compel you to change them. You would be best advbsied to read the lease, but this is the general position. If there is a problem with what you have had installed then they really need to give you chapter and verse as to why - if there's a defect does your own installer have any liability?

Finally, this may fall under s20 expenditure, so a consultation would be needed, depending on the cost.

What does your lease and your contract with the management company say? I should also detail your agreed procedures in the case of disagreement.

Am trying to picture what's involved with replacing the window waterproofing that requires your windows to be replaced. I think the windows are removed & once the waterproofing corners & membrane are installed it will mean the gap within which the window fits will be a fraction smaller than before, hence new windows. BUT... I would have thought this applies to all flats' windows old or new as the size gap will be a fraction smaller for all of them and therefore they'll all need replacing, so why do you have to pay for yours? With regards can they insist on this waterproofing, it depends on 1) how watertight are your current windows 2) even if your current windows are proven to already be watertight & not need redoing, will the different windows not be in keeping with the other flats and the lease allows the freeholder landlord to object to this or you need retrospective planning permission 3) assuming this cost is more than £250 per flat, it would require a Section 20 consultancy process - what works did the S20 say was included?

My guess is, the anticipated yearly budget has come in way under. After a quick word around the agents office one of the new property managers partners has a upvc window business. Regardless of serving an S20, the agent uses said partner. A nice divvy up on the invoice front and it's all water tight! Which was the original plan.

It is difficult to comment fully without knowing the construction of the balcony relative to the windows. If the windows rest on the balcony and the waterproof membrane goes under the frame(s), then the windows may need to be removed for the membrane to be effective. However if there is no physical contact between the two parts I cannot see why the frames need replacing. Assuming that the frames need to be removed, then the contractors probably don't want to be responsible for damaging the frames on removal, or that they won't fit when re-installing them. you need them to explain and to comply with Sec 20

Hi as a architect if whatproofing the deck you would normally take the water proofing under the cill of the doors and turn it up. - thats so water running down the windows and reveals drains out even when windy
Your windows should be removable and sufficient gap.
If there is a 150mm step down to the deck then above not so necessary. I presume you put a cill tray under your new windows when replacing them. Too often builders relie on a bit of mastic around the frames but if exposed it often leads to water ingress over time
The other thing is did you get planning permission for the replacement/change and landlords consent. Yes even a like for like change requires PP. If you got LL consent then unless your windows are defective or not installed correctly then cant se how they can require you to replace them.