Sorry mrct, going to go against the grain here.
If you centred a
tile in the middle of the wall instead of the grout joint, then the cuts either side of the window would have been full and not split. (This looks possible because your current cuts either side of the WC look less than half a
tile width) It would have looked better that way (IMO) and also would have been quicker and easier to
tile (despite having to plunge cut the flush plate)
Another option and as a half measure you could have tiled the bottom like you have and then still done full cuts either side of the window. (not something I would do but acceptable I suppose) Although not technically correct, visually it would still look better than it does now, where your eyes are not drawn to the off center window.
Sometimes I think tilers are too hung up on centering windows or walls because that is how they've been taught to set out a room, but by default dismiss the overall visual appearence. Setting out tiles shouldn't be about '
rules' as it is still a decorative surface. Follow the rules, but if they don't work, adapt them.
All that said I don't think they should withold payment, at least not all of it. It looks to be a very tidy job you've done there. Personally I don't think either you or the customer is in the wrong, just a difference of opinion in how it should look. I suppose the fair outcome is for both of you to meet in the middle. Say it costs £300 to redo how the customer wants it, then pay half each.
JMO of course :lol: