Is it  possible to test a central heating pump for efficiency as I'm not sure  if ours is pumping round the system as good as it should?

The  reason being the rads have never seemed to get really hot. But during  the heavy snow last week, two of my rads stopped working compeletly.  They were working the day before.

I bled the rad, opened the lockshield valve water came out, opened the TVR and nothing. Thought either blocked, frozen or both.

Up  into the loft space, warmed the pipes, opened the TVR, at first a  trickle then a small splurge of mucky water then clear. After connecting  back up, all the rads got hotter than they have ever been. It stayed  like this for about a day then back to normal. Just the odd time they  get very hot. Also the boiler knocks off quite soon after being switched  on, that's what made me think it might be the pump as it might not be  pumping the cooler water away from the boiler quick enough?
Thanks for any replies.
Malc

Yeah the insulation part is now sorted. But sometimes the rads are really hot and other times they are not.

Sounds more like a dirty system than anything, sludge or air.