So....this is what I ended up doing.
Hosepipe section in set into floor, correct distance out from wall, terminating underneath bath. (end taped up!)
Sensor pushed in right to end, then back 2 inches so not under tape.
Rest of sensor wire set into floor and into wall, terminating nicely inside back-box for Thermostat.
Thinking behind it is:
All documents say its OK to extend sensor wire, couldn't find anything specific to this item that says it can be cut.
So, if it fails, it can be withdrawn from under floor inside bath panel, and replaced and joined to wire running to back-box.
Simple!
I ended up getting mixed replies from Electricians Forum, one fella said my plan sounded fine, other said put it in conduit all the way! (wasn't an option unforts!)
Thanks for all your help...
Pictures to follow when its all done. xx