I'd guess ours needs doing every 3 months or so. The first sign that it needs doing is that after the boiler has been heating the water, the hot water pressure is a bit higher than normal when turning on a hot tap - as soon as the tap has been opened the pressure is back to normal again.
Hmm, if a pressure relief valve is either leaking or operating then I'd get someone qualified in to check it over. Those valves are there for safety reasons and not to be ignored...
Edit: I'd assumed you had tried regenerating the bubble without any success, if not then give that a go before calling anyone in!
I can't see how adjusting the tap isolation valves should affect the pressure in the cylinder. The only scenario I can think of is if your cold water pressure is higher than hot, and the your tap is somehow allowing the cold feed to pressurise the hot above the point where the pressure relief valve operates. (But a main point of a megaflow is to normally to have your hot water at the same pressure as the cold, so unless it's not installed correctly or you've got really high cold water pressure and there's a pressure reducing valve in play...)
Are you absolutely sure that the leaking overflow is from the megaflow? The pipe from the temperature & pressure relief valve (the one you turn to let air into the cylinder while regenerating the bubble) should go via a tundish before the overflow pipe to outside. Can you check that this is where the water is coming from? There is a second pressure relief valve in the cold feed valve which should have a separate overflow.
If you've regenerated the air bubble and the overflow is definitely from the megaflow, then there are two possibilities - either a pressure relief valve is faulty and leaking, or there is a genuine overpressure. Either way, I'd get an authorised company to come and check it over asap.