"Who is responsible for this bill now? "
Well, there's a pretty picture.
So:
1. There is a Management Company.
2. You, the Vendors, did not know there was a Management Company.
3. Therefore, unless it was totally obvious from the Title, your solicitor did not know there was a Management Company.
4. The purchasers' solicitor did not know there was a Management Company.
5. During the conveyancing process, the query as to whether the Multi Unit Development Act applied will have been raised and, presumably, the answer was no, because no-one knew there was a Management Company.
So, the Million Dollar question?
How obvious/clear was it that there was a Management Company involved?
Personally, I would have thought that the vendors should know this. Or that it should be clear from the Title Deeds/planning permission. Or that the purchasers will have ascertained this , right at the beginning.
So, go back through that thread- did the vendors know? Is it obvious? Did the vendors' solicitor act on their clients' instructions? And where, ultimately, does the liability lie?
And it may well lie in the vendors' lap.
You might want to carefully check all the details. Only then can it be ascertained where the original misapprehension arose.
mf