Worchester greenstar CDi 35 losing pressure?

Hi, I`ve had my Worchester CDi 35 for 1 year with no problems, runs great however last month I`ve been having to top up the water. Under normal conditions the gauge reads 1.5bar and under normal working conditions rises to 2bar then returns to 1.5bar on idle however I`ve had to top up once a week I was advised to check red tank ?? ie shredier valve which I did air came out no water what else could there be wrong. There is no evedence of any leaks in boiler or pipework unless there is a leak under the floor ?? but the system has been fine for the past year, any advice appreciated.