Ok Guys update! I've been busy with other projects but have came back to this now. (6 months later!)
The floor is dry as a bone! I got 2 local plumbers to have a look. One guy thought there could be a leak but couldn't be sure! The other (trusted long term plumber i use) said in his opinion there was no issue with the floor n to get a floor down again!
As for the boiler using water issue he suggested trying a stopleak treatment into the system as a potential easy solution. Which I haven't gotten around to doing yet.
I have my mum talked into a laminate floor for the ease of lifting if ever required. Last thing i want is to retile and maybe having a problem in the future n tiles needing to be lifted and destroyed.
I have removed all the tiles, which has left adhesive covering the screed. Seems to be firmly adhered to the screed. My plan was to remove old adhesive if required and self level.
Removing adhesive, i bought an sds drill n wide chiesel. Is this my best way of cleaning it off?