Update

Turning off three out of the four, the rad eventually gets warmish...nowhere near as hot as normal rads elsewhere in the house.  Turn that rad off and another one of the four on and the same scenario.  Repeated for the remaining two. With all four rads on, they all are tepid.  The temperature of the return pipe at the boiler is hardly warm.  Certainly I can comfortably hold it.

Is this pointing to a blockage in the main 15mm pipe?

Rad A gets hot.  So too do all the other rads in the house (not shown).

So this morning, fired up the system from cold. Rad A and all others off. Rads B > E trvs off (have checked plunger is free to move and in open position), all lockshields fully open. Started pump and boiler. Went up into loft to watch level in F&E tank. Level in tank gradually rose (as water heated up presumably),  No sign of rads heating up. Lots of noise from the loft. Went up to check to see pumping over.  

Turn on some of the other working rads and pumping over stops.

So this suggests to me a major blockage in the 15mm pipe (flow or return?) between Rad A and Rads B>E?  But don't understand why pumping over should occur since the vent is on the negative pressure side of the pump.  Unless the pump is fighting against a blockage and one can get back pressure in the pump?

So now I am (a) totally confused and (b) fed up.

Left the system running with rads B > E open. Rad B got tolerably warm. If I held my hand on it for a minute then it was starting to get uncomfortable.  So this turns on its' head my earlier conclusion that the flow to rads B > E was blocked.

OK...turned off Rad B hoping to encourage flow through C, D and E.  All that happened was that the pump starting sounding as if it was churning the water against a resistance...which it clearly is since none of them got anywhere near warm...although yesterday I managed to coax a few degrees out of them.

I have run out of ideas.  Time for a professional ?

Bloody hell...you're good, mate !

In between posts I took off D and E and did what you suggested.  Loads of black crud came out..well, 1/4 cup a a guess. No idea if that is normal or not.  These are low-volume rads with narrow pipes so that can't be good.  gave the rads a lot of shaking, swirling, flushing until it ran clean but reckon there may still be some residual.

Feeling optimistic, I reconnected and hoped for the heat to come blasting out. It did not.  The rad got warmer than it has in the past.  Left B on all night in the hope that it would improve.  NB this was the only rad ON. All others turned off.  made damn all difference.  I reckon that sludge remover is a con. I can't see how it can possible work if the pipe is blocked good and proper.

So I've hired a power flush...should be here tomorrow.  And I'm going to fit a MagnaClean.

Oh yes..total of 17 rads ..all low volume.  This type  http://www.diy.com/diy/jsp/bq/nav.jsp?action=detail&fh_secondid=9271408&ecamp=tr...

No, it's a Kamco.  I'm fitting a MagnaClean which is on order.  Any tips on using a Power Flush.  Rads B to E are stubbornly refusing to open up.  I know that D and E are pretty free as I removed them and flushed them out and the water flows OK.

It seems to me that if the 15mm pipe is blocked then no amount of power flushing is going to shift it.

Some progress and a lot of steps backward.  Mid-morning the powerflush threw one of it's main pipes and flooded the utility room with cleaning fluid...fortunately I'm using Sentinel X800 which they told me was citric acid based and so a good wash would be sufficient.

I believe I have three problems.

suspect blockage between rad B and C

suspect blockage in feed to D and E from the junction where it splits to B and C

Definitely known major as in terminal blockage of the flow from rad A to these four.  The pipe has to be replaced, it is so bad.  And to be honest, I reckon powerflushing and all these chemicals is a con.  Sounds good but seems to do damn all.

Just to update. I replaced half of the 15mm pipe (not microbore!!) from Rad A to the others. The rest would need taking up the fitted carpets etc.  Magnetite seems to love sitting in plastic pipe but not copper.  Used a long piece of curtain wire to ease through the magnetite in the remaining half.  

Also had problems downstairs with a couple of rads (immediately below Rad B). Used the curtain wire dodge again and this time (as it was open joists down to the earth) just let some water flush it all out.

Ran the power flush...all the rads got warm (eventually...think that I might have to (if SWMBO agrees) remove rad B and C.

Powerflush sprang its' hose again. Discovered that the muppet in the hire shop had wrapped black slippery duck tape around the ferrules. Thus ensuring a perfect sliding joint...slippery duct tape against hose.  Got showered, face full, some splashed in my eyes, elm surrounds wrecked.  In discussion regarding compensation.

Following a prompt by The Scruff, I took out the central magnet part of the MagnaClean and let it dangle in the water of the powerflush. Within seconds it had acquire black magnetite.

So cut a long long story short...system up and running ..although a couple of rads still reluctant to heat up fully. Will give them a stern talking to tomorrow.

Thank for the advice and suggestions, guys.