Thanks for that Tony
Its a true system boiler, Ideal Standard Mini S24, on a sealed system.
I think i've sorted the pressure problem, the fill loop was connected and appeared to be off, but was actually slightly open. The valve is faulty and will need to be replaced, but i've disconnected it and capped it for now.
I've checked for any obvious restrictions in the system, and have largely ruled out air as being the cause. All rads heat up fine, with no delay, they have all been bled also. The cylinder and DHW system have had air bottle valves fitted also, i've loosened the upper connection at the cylinder coil just to be sure, but there is definetly no air.
The CH flow switch on the boiler does work differently at start up in CH and DHW modes. When DHW is selected only, the boiler goes through its start up procedure (ie starting the circulation pump), and generally will not fire. In this instance, the CH flow switch only opens about half as much as it does when CH is selected only (and it then in turn fires.)
I cannot comment at present on the flow and return temperatures, and will get this checked. The only other comment is that the sytem has workede fine for well over a year, and decided to malfunction. My understanding is that there would either appear to be a restriction (either in the cylinder coil or possibly the motorised valve, although i've disconnected this and checked it), or that the flow switch or another boiler component may be faulty, due to prolonged operation at high pressure.
I'm happy to get someone in to have a look at the system, but i'd rather have a fair idea of the problem first. A chap called in last week and wanted to rip out the motorised valve, thermostat and timer (and boiler if i'd let him) and replace with a lever valve, and had only given the system a glance. I have to say, I had to ask him to leave!
Any advice would be welcome