Pressure dropping in Valliant boiler

[littlesilver]

We've noticed over the past few months that our pressure keeps dropping - it has never stopped working but when we think to check it is almost zero. We top it up every 3 or 4 weeks.
Had British Gas out today for annual service so mentioned it and the guy checked all my rads for leaks and found nothing. He has suggested it is my underfloor heating which is just in my kitchen extension - I only had it fitted 3 years ago and am obviously reluctant to go ripping the concrete tiled floor up.
If I get an underfloor heating engineer out for a service will they be able to tell if there is a leak without ripping the floor up?
Thanks

 

[BingoBongo]

top it up to 1.5 bleed the system and stick the heating on for a bit

check the pressure and see if it rises to 3

see if the pressure relief valve is dripping outside.tie a bag to it and check next day incase its a small drip

check for leaks at rads when system is cold