*UPDATE*
Ok, so a new plumber came in, replaced the pump and checked the filter on the return valve (which was clear of blockages).
Whilst it initially appeared that the new pump was working (and in fact it is working - it's firing away once it drops temperature, which it wasn't doing before), the rads upstairs and downstairs aren't working as they should be.
Let me clarify - the downstairs rads aren't heating up at all, even though the flow pipe from upstairs to downstairs is scalding hot! Non of the flow pipes to those rads have any heat in them. Upstairs, all the flow pipes to the rads are burning hot. However, whilst the batchroom radiator and the one in our bedroom are 'warmish' (thermostats and lockshields are fully open), the rads in both the boys rooms are cool to the touch i.e. little to no heat at all.
I've told my plumber that I'll be monitoring it over today and tomorrow, but to be honest, that fact that the downstairs rads are stone cold (they've all been bled and are full of water), clearly a new pump has only solved a part of the problem.
Incidentally, I checked the two 22mm copper pipes coming out of the boiler (flow/return) - one of them is scalding, the other is very hot. The return pipe from downstairs back to the boiler is cold.
If anyone has anything they'd like to add to this thread, something I can relay back to the plumber for him to investigate on his next visit, please let me know and I'll pass the message on to him.
Cheers!
