Outrageous! I’m assuming you’ve spoken to your LL to discuss the situation? If you have and she will not compromise then I would start the complaint process.
When I moved from my last flat, the managing agent tried to charge me for various items including a ‘light’ cleaning, even though I had taken a day off from work and scrubbed the place from top to bottom, including the oven which was wretched when I moved in.
The discussion to come to some sort of agreement did not go well so I sent an e-mail to the agent stating that they should not to return the deposit until the issues had been resolved and that I would be making a formal complaint to through the rental deposit scheme. Before I could follow up with a written letter they agreed to refund the entire deposit.
This was my second time renting through this particular estate agent and until this incident found them to competent and fair.
I think you should stand up to your LL. £800 is a lot of money to give away. She’s entitled to something since she did clean the flat but not that much.
Whatever you do, don't accept a partial refund if you plan to take it furrther and make a complaint.