Replacement of hidden cistern quote

I've had a quote of £830 + VAT to replace a cistern that is located behind some tiles in my bathroom. You can get an idea of the layout from the attached video.

I've added some text below from the company who have provided a quote to carry out these works:

^(The cistern tank along with the flush kit will need to be changed. You can see by the attached video that the access is very limited. There are only 2 ways of changing the tank, which is to break open the tiles at the front behind the toilet which is very messy and it would be difficult to retain the tiles.)

***FWIW The access from the hallway is not possible, so they then provided this follow up and quote****

^(Our quote is to attempt to cut a large removable panel behind the toilet. We cannot guarantee that we can keep the tiles intact. If the tiles crack then we will need to replace the tiles for the removable panel which may not match up with the rest of the tiles. The panel will be large enough to access the entire cistern tank allowing for the cistern to be replaced. We will supply and fit a new cistern, with matching flush kit and button. The panel will be secured by Keku fixings and the grout lines with silicone sealant.)
^(The total charge is £830+VAT.)

I'd like to know others plumbers thoughts on this process, and if this price is within a reasonable range for these works in a London property as I have no idea.

Thanks.