Thanks for all of the help
Thought I'd put up a photo to show what it looks like. The whole tiled area is only 2m x 1m. It's really just used as a cloakroom but happens to have a bath with
shower over, in the room too. The hairline cracks are really hairline - wife can't see them but they are definitely there and there is the tiniest movement up and down if I press a couple of the tile corners. Cracks are only across the 1m width too. I'm thinking best to leave for now but take up tiles when they do come loose and screw down chipboard into concrete floor underneath. Then use more flexible
https://www.tilersforums.com/forums/tile-adhesive/ and grout. I think I'd have probably been OK if I'd gone for S2 and a flexible grout in the first place. The Ultra Tile Fix ProFlex SPES S1 supplied seems to be like concrete with zero flex and the Mapei UltraColor-Plus grout doesn't mention being flexible at all.