Radiators not hot enough

[newbiev]

Update:
So it is a 2 pipe system.
The feed pipe is scorching hot - can barely touch it.
The return pipe is hot - Can touch very briefly before it hurts
When I bleed most of the rads, the water that comes out of the bleed valve is clear-ish.
When I bleed one of the new rads downstairs, the water is a dark/brown black. I collected this in a bucket, and there were actually small particles of something in it which sank to the bottom of the bucket.
The boiler fires up and down even when desired temp has not been reached.

Does this indicate the need for a power flush?

 

[Daveydub]

The return pipe is hot - Can touch very briefly before it hurts

The boiler fires up and down even when desired temp has not been reached.

The boiler shuts down when it is up to temperature (Return is hot) so it is circulating around either the cylinder or the "easiest" rad and going back to boiler.
Is it any better on heating only?
Is there a rad that gets particularly hot?
Is there a by-pass, perhaps in the airing cupboard?