To all who have replied so quickly, thanks so much. Here we go:
I live south coast 5 miles west of Brighton, in Lancing. I bought the tiles from 'Tiles Tiles Tiles in Portslade. They are porcelain but I don't know what grade. They are a whitish colour finished with 'sparkly' bits ingrained and have a mottled feel to them.
They were laid with the recommended adhesive and grout (by the same store we bough the tiles from), then he rinsed them 6 or 7 times, ironically using a new sponge and bucket he'd bought that day - the tiler was brilliant. We know he didn't use dirty water or sponge etc.
The next day we knew they weren't right. They have taken on a yellowish hue on about 95% of the surface area. We cleaned them loads of times with the recommended LTP product following their instructions to the letter, then again with their more concentrated LTP product.
We cleaned them again and again but when you place the new tile next to one, you can see they simply don't look the same. the grout is fine and looks the same as when applied the first day (April this year)
Thanks again for your help
Dave