Being realistic mate, the reason its leaking is because you've removed and refit the sanitary ware, assuming that it wasnt leaking before, even if it was, your not going to be able to prove it, The number of times i've seen perfectly dry fittings leak after a removal and refit, they can be perfectly fine and prehistoric, but as soon as there undone and redone they leak. This is why I dont touch plumbing.
I appreciate that you want your money, but you shouldnt have touched the plumbing, no matter how easy it seemed without it being covered on your insurance. She is well within her rights to bill you for all the repair work, plumers bill, plastering the ceiling, painting it completley, the job lot.
Stopping the cheque is a big no no in the courts eyes and if you want to take her to court to claim the payment you can, but unless you pay the bill or rectify every last bit of damage caused she could just counter sue and get the cost, which she could easily make more than your tiling bill if she wanted to by adding the plumbers bill for the repair, a new ceiling plastering and painting, Granted she should be allowing you to arrange the rectification, but would you if you were in her shoes, how many times do people post on here unhappy with work done for us to simply say 'kick him off the job'. (Im not implying your some sort of cowboy by the way).
Youd have to ask the plumber why hes cut a hole in the ceiling but im guessing he was trying to do you a favour by not ripping up the new tiles, and hadnt noticed the leaking valve initially so was investigating the pipework under the floor boards, but thats something you'd have to check with him.
Im not trying to be harsh but you've put yourself in a very difficult situation by undertaking the plumbing work, which has leaked no matter how simple it seemed.
Not to mention the damage to your reputation if she decides to shout about this from the rooftops.
If it were me I would be getting the work sorted by a qualified plumber with liability insurance, along with the ceiling repair and taking any money thats left and trying to salvage the situation walking away with a reputation for making sure she was completley satisfied in the end.