jezsoup,
The baseboards you refer to will be useless - they are only 9.5mm thick and will introduce more flexing than you already have.
You need to assess how stable the floorboards are at the moment and fix them down so that you cannot detect any movement as you walk on them. You may need to reinforce some of the joists to give a stable floor, especially if the bathroom has been refitted in the past.
If you cannot achieve a stable floor you will probably have to use 18 or 25mm thick WPB plywood.
Stick the tiles down using one the special adhesives which include latex.
end
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