Myson Compact CP61 pump issue?

I have the above mentioned CP61 pump installed in a Potterton Puma 100e boiler and I am not sure whether it is working correctly. Radiators closest the pump seem to be very warm those furthest away less so. Radiators have been bled, TRV pins freed,etc so problem seems to be maybe more pump/sys related. I've tried to check whether the pump is working correctly but am not sure if I am checking the right thing as per the user guide. One interesting point is that sometimes the system works fine(and all radiators heat up to a certain extent)other times it just seems to stop.
When I turned the black hexagonal nut on the pump it came away(it's plastic) and there is a screw behind it. I have twisted this and some fluid comes out...should I twist the screw the entire way out to check if there is maybe a problem behind it or not? If so then what do I need to check?
Also, on the Potterton should the green flame light be lit all the time the CH/HW system is on or not(it seems to click on and off from time to time).

Thanks for any help in advance.

COuld be the pump going slow sometimes, or the diverter valve sticking. Hard to diagnose by remote - I'd use a thermometer to see how much hot water is going where. If you fancy changing the pump head it wouldn't be wasted effort - they don't last forever, and aren't too expensive if you know where to go. You have to drain enough water, but it's a straightforward job.

Is this using a thermometer on the rads or on the pipework in the boiler housing?Is the only real solution to change the pump head or can I check a few more things before going down that path?

I had another go at unscrewing the screw in the middle of the pump yesterday...it doesnt seem to come all the way out(maybe thats how its supposed to be) so I couldnt get as far in to see if there is a need to get the impeller going. However when I screwed it back in and put heating back on it seemed to start(could hear the pump going OK) and the radiators seem to be heating up OK this time(except the closest to the pump this time...it was a bit lukewarm)

thanks