Guys, thanks very much for exchanging comments as it is way beyond me but i think you seem to have come to the same conclusion but as Rob has said, the cold comes in to the valve at the top and the cold balanced out immediately of that valve to the left / behind the valve.
However, what i can highlight having kept trialling things:
1. The same issue of warm out of the cold tap happens in every cold tap, bathroom, ensuite, kitchen etc
2. If I run a cold tap and go through the 30sec process of getting rid of the warm to eventually ge tthe cold constant supply, if i then open another tap in any part of the house the warm that then comes through is only for a few seconds and far less than normal.
3. Whilst the warm is coming out of the cold tap, if i then turn off the cold, and turn it back on, and then do it again, it seems as though this stpos the warm coming through far quicker than waiting the 30secs. The only problem is, i don't know if it was going to change to cold anyway but it defiently seems to speed the process up.
4. As Endaxi asked, if i run say a shower, the cold taps then tend to run cold immediatley and no warm water comes out at all
5. But if i run a hot tap for a short while, therefor enot usng as much hot water as a shower, this tends to speed up the process of the warm water not coming through for as long when the cold tap is turned on, although it still happens.
6. By the time i check the cold inlet pipe in a morning it is very very warm / hot. This was 26 degrees when tested the heat of the pipe the other day. I assume becuase UFH, CH and DHW are all on so making it work hard. However, this morning I got up early so all of these had been on but only for about an hour. The cold inlet pipe had not got warm. I ran the cold water tap and this still did the same thing, 20/30 secs of warm water. The pipes above the combination valve weren't freezing cold and even though the cold tap was on, they did not turn freezing cold until about 20secs later, at which time there was still warm coming out of the tap but then soon stopped. However, the cold mains supply pipe coming out of the floor wit the sto pcock on, wnet freezing cold once the tap had been turned on. Could this mean that the water between the stopcock in the cylinder room up to the combination valve is warming in the pipes as i think that pipe runs up the wall cavity, across the ceiling and then out throgh the pipes you can see coming out of the wall.
I don't know if that helps with anything but thought i would explain in case it made a difference to any of your thinking.
Thanks again as always