Right... For anyone with a Megaflo cylinder, particularly one that was installed many years ago, that has similar problems of warm water from cold taps and/or frequent problems with dripping taps and by-passing toilet cisterns, and is reading this thread for the first time, I will edit the opening post to direct you to this one where I will attempt to summarise the symptoms, cause and cure of the problems...
My Megaflo cylinder was installed 15 years ago and was designed at the time so as not to have an 8 bar pressure relief valve and check valve between the cold water inlet to the cylinder and the tee to the balanced cold water supply. The tee to the balanced cold water supply and tank inlet is just after the cold water combination valve that reduces the pressure to 3 bar maximum, provides a check valve and currently houses the 8 bar pressure relief valve.
The air gap in the cylinder has never been knowingly re-generated as should have been done annually. The depletion of the air gap over time has meant there is less expansion room so there has been a resultant increase in pressure in the tank when the when the water gets hot (although not enough to cause any visible discharge through the tundish).
The resultant high pressure has been felt through the whole of the hot water and balanced cold water system which includes all water outlets (including toilet cisterns) except the kitchen and utility cold taps (which are fed direct from the incoming mains). This high pressure has probably contributed to the many dripping taps and toilet by-passes that have occurred over the years. As one was fixed another would start to drip.
Recently, during the replacement of the kitchen tap that was dripping, the hot water was drained sufficiently from the Megaflo tank to inadvertently re-generate the air gap. This has caused the pressure in the tank to be less than it was previously when the water is hot but has allowed the expansion of the hot water into the air gap as is supposed to happen. Suddenly, a previously dripping hot tap in the bathroom has stopped dripping.
The downside however, is that once the water has heated there is an amount of expanded water in the tank that would release with a short burst when a hot tap is first opened. This in itself is not a major problem. Without the pressure relief valve and check valve in the correct place however, if a cold tap is opened first (before any hot tap is opened or a toilet is flushed) then that expanded water will fight against the 3 bar balanced cold and force its way out of the cold tap until the hot and cold water pressures equalise. The cold water will then run from the tap as it should. In my case, the water from the cold tap could be as high as 60c for around 30 seconds before gradually getting cold, so quite dangerous if you don't expect it.
Pressures now equalised, everything will work fine until the hot water expands again, particularly if it has been re-heated, and the process repeats itself.
So, re-plumbing the system to put the pressure relief valve and check valve (NRV) between the cold inlet to the tank and the tee to the balanced cold, and re-generating the air gap periodically, will fix the hot water from the cold tap problem, keep the pressure at the balanced cold taps at 3 bar max (as per the 3 bar pressure reducing valve in the combination valve set) and will help keep the water pressure at the hot taps from being un-necessarily high, therefore hopefully reducing the number of issues with dripping taps.
I have learned that it's important to get the pressure relief valves checked annually for correct operation and that an air-gap re-generation once a year on top of any necessary re-generations (water coming through the tundish for instance) is also no bad thing.
A fascinating journey to this logical conclusion.
If I have made any errors in this appraisal then anyone can please feel free to correct me and I will amend it accordingly.
Thanks to all you generous people who have given their time and effort to help me reach a conclusion to the problem(s).
