Many thanks for all your comments, they've been extremely useful and I'm very grateful to everyone who took the time to look at the photographs and express an opinion.
My wife and I were horrified by the state of the tiling (and by other aspects of the job, especially the way the cork floor tiles were laid - they're lipped and there's a ridge under some of them, right by the door, and we're certain they're going to have to come up and be replaced). We needed a reality check from people who know their stuff, however, and you've certainly provided that.
As requested, I've taken four more photographs, general shots of the bathroom to put the detail into context, and have added them to the bottom of the web page.
The reason I particularly wanted comments before this afternoon was because the guy that owns the company who supplied the fitters is coming to take a look at the work and I was expecting him at any time after 12 o'clock. When I spoke to him on the phone yesterday his attitude was that he'd been there in the afternoon and the work had looked fine to him, so I was expecting him to attempt to bluster his way through my objections. He phoned a little while ago to say that he now wouldn't be able to get to me until tomorrow afternoon as he's very busy. I told him that that was totally unaceptable, that time is of the essence and that all I needed from him was his official statement as to whether or not he found the work acceptable and I'd then have no further need of his involvement as I didn't want any of his employees involved in the remedial work. As his company was sub-contracted to do the work by Wickes, who provided the bathroom furniture, they will provide other fitters to put the mess right. His attitude changed somewhat on hearing that and he then said that he could still make it today but that it would be quite a bit later.
Wickes have been very good, assuring me that we're covered by their guarantee and that as soon as the individual in question has had his say their contract manager will come and look at the work, give his opinion and authorise the remedial work as necessary.
Hillhead > I live in Yateley, in N.E. Hampshire. Hopefully we won't need anyone to come and take a look in person once Wickes' contract manager has examined the work but thanks for asking.