I suppose I should know as I have an unvented cylinder "installers" CITB certificate but I'm not sure maintenance demands the same bit of paper. As long as renewal with like for like is involved and you dont compromise safety critical components what's the problem?
No one in officialdom will know unless you tell them.
Most of the unvented cylinders I've met have a diaphragm type "cushion" vessel but the principle is the same. I'd be looking for a restriction on the input as first port of call.