After the leasehold on new houses scandal and the seemingly unfair maintenance charges on new housing estates far more people, and solicitors, are awake to the impact. My daughter is on the first time buyer trail and isn't even looking at new houses or houses in unadopted roads - why place your future costs in the hands of a for profit company. That might make it very hard to sell, unless it appeals to a specific market which is under supplied with property, which retirement/sheltered might be. If there are other flats for sale nearby at lower charges, you have a problem.
A tactical decision, is it wise to start a potentially public complaint, which you might have to declare under 'buyers enquiries', or sit tight, keep quiet and get it all over with ASAP however unfair it might be.
Although fees seem high to you, the worry-free aspect might have been of high value to your mother, hard to measure in £££ but it wasn't all wasted money.
If I were buying I couldn't even begin to calculate the discount I would try to negotiate. 3% off to start with because that's what I would lose when I sold, then from the sound of it 7200 a year for services I might think are worth half that, so if I live 10 years that's 36k I want to knock off... I know it doesn't work like that but it's a starting point. But that's me, in good health and with a decent knowledge of electrics, plumbing, building maintenance, garden etc. If though I had a decent retirement income and wanted to not worry at all about the building, or about my surviving partner having to worry when I died, etc etc I might think it worth it.
Some retirement complexes had a put option, you could sell it back, losing shed loads of value but at least stopping the outflow of cash. Unusual, but check. The agreement might have something about what happens if you default on fees. It must happen, people die intestate and with no assets, empty flat no income.
Finally, if the rhyming ship stopping company is a housing association, they are regulated. Used to be the social housing regulator, probably renamed. Some though have a regulated and unregulated business arm. Might be worth digging around.
Sounds like rock and hard place, trying to contact other residents or other vendors might be the most helpful first step.