scratching my head....

[IDCHAPPY]

Take the radiators off and open the valve into a bucket to see if you have any flow of water, if there's nothing then it's probably gunged up.
Is it microbore by any chance?

 

[Leo21]

As above, check both valves flow. Don't forget boiler will need toping up. If the pump is I side the casing, no your not aloud to change the pump.

 

[T & H Plumbing]

It's a combi Valient ecomax

It's coming up in 15mm

And yes the pump is incased

 

[IDCHAPPY]

15mm should be ok usually. Still worth checking the pipework and radiator before boiler pump.

 

[T & H Plumbing]

Thanks chappy ill do that first thing just a bit gutted I couldn't sort it today tried everything

 

[TFGplumbing]

If the pumps okay, You could also check that the Diverter is not bypassing to PHE when its on heating, effectively taking the ''ummmff'' out of the pump and not letting it get to the furthest rads.

Rad valves are always a good bet