You may laugh but the going rate for the same flat as mine in the building seems to be around £1k+ a month, which is crazy to me but people do pay it. That's why I'd be happy to let it go for less than market rates if it meant a quick deal was there to be made! Would the costs of running the flat, i.e. mortgage, bills etc not be tax deductable? Apologies for any daft questions but I've never had to manage my own taxes before.
When I last spoke to the mortgage company they just told me that I wasn't able to rent the flat because it was on a help to buy scheme, so not sure if going to them for advice would help?