Happened to a friend with a flat in The High on Streatham High Rd back in the '90s. The roofs were rotten. Turned out the facilities managers gave the work to friends, getting "mate's rates" along with an invoice for the supposed actual cost, then ripping off the leaseholders. Bill of £14,000 per property which suddenly went down by 60% when the managers got rumbled (and sacked). If they'd got away with it, they'd have banked about a million quid from the scam.
Always worth pestering for a full breakdown of costs, and don't let agents put you off - you have a legal right to see the costings and accounts.