ideal classic ff280 pressure problem,help?

when i top up the pessure to 1.0bar and dont use boiler the pressure will remain ther.go,s back to 1.0bar when using hot water,however when i put heating on pressure go,s to 3.0bar but then after an hour or so pressure drops reading zero.only happens when using heating.some came round and said need new expansion vessel and new calorifier.would this be correct