Boiler man came and looked today, replaced the prv it was leaking some water onto the vessel and replaced another part ( unsure if this was leaking!, I wasn't there so I'm hearing second hand info from my mum!)
Boiler guy reckons this has been the cause of the pressure drop but to monitor it. What it has been using over the 4/6 weeks he thinks would maybe be a cup of water so very little.
He doesn't think there is a leak in the floor as pressure drop is so small, no visible sign of water on floor/ up walls, heating is working perfectly.
His opinion is the floor has lifted because of the poor tiling job which is of course is definitely correct! Just still seems strange how it lasted so long then tented so badly in a straight line over a period of approx 2 weeks.
So! Monitor now and hopefully this will put an end to topping up the boiler every 4/6 weeks