If you buy this flat, subletting fees are £10,000 … investors deterred from buying

This is from Heather:

Hi, I’d like to know a couple of things.

We tried to buy a flat as a buy to let, we were later told the owner of the block had a clause saying properties could only be subletting after a deed of variation in the lease, for which we were told to pay £10,000 as the flat’s owner didn’t want to pay.

The flat owner put the flat on the market again and we got a letter from the estate agent saying it could be as low as £1500 or £1000 but we had already missed out by then.

The flat was being let out and the same estate agents who tried to sell it to us were managing it on behalf of the owner AND had originally sold her the flat, yet they claimed to know nothing about the clause prohibiting subletting.

I am trying to claim compensation for lost fees. Any ideas?