I think that's a bit of a personal question to be answering on a public forum.
It must of been a price he/she felt was resonable in the first place on the provision the work was up to
standard. I thihnk the critical thing now is getting the tiling sorted at no extra cost to yourself.
Get a couple of quotes to re-do the work by professional and recommended tilers. Then put them to the installer you had originally. I'd go along the lines of "put the job right at your cost or write me a cheque for £*** so i can get a professional in to do it"
Good luck. Be interesting to see the result once sorted, post some pics and keep us updated.