Worcester Greenstar 24iJunior Combi boiler?

Have a Worcester Greenstar 24i Junior Combi boiler recently fitted. We had a problem where the pressure kept on dropping, the downstairs radiators were isolated as the pipework runs in a screed floor, the pressure stayed up, we also noticed shortly afterwards, firstly in one area then a few days later in another area the skirting starting to pull away from the wall, this i presume was the leaks in the pipework, strange how we never had any problems with the old boiler before. The pipework for the downstairs radiators now runs along the surface, some in the garage which has been lagged and boxed in.
The boiler seemed okay for a while, but just lately the pressure has started to drop down to zero again, we can't see any visual leaks. I will be checking the pipework in the garage tomorrow to see if there is a problem.
Another problem we have is the hot water in the bath, it seems to take ages to fill up, the plumber who fitted the boiler said it should flow at 12 litres per minute, i have down a test myself by turning the hot tap on full then placing a bucket underneath and removing it after a minute, i then measured the amount of water in the bucket, the result was 7 3/4 litres, quite a bit down on what it should be.
When we mentioned how slow the hot water was when he installed it, he told us what the flow rate should be and being certain it was flowing okay but didn't check it, other than that he didn't have any idea what could be wrong, by the way the boiler has been fitted by a Gas Safe qualified plumber.
I know i can't touch the boiler, but is there anything i can do to eliminate anything, i will do a flow test on the cold water to see what this is, could the water pressure to the boiler be too low? this wouldn't make the boiler lose pressure?
Any ideas what it could be, i have another plumber coming round but it would be nice to have some sort of idea what it could be first.