I'd throw a Spirovent Air Eliminator on it. i had tremendous air problems with my system at my last house. Put in the SV and 24 hrs later there was no air. I wonder if your system is "reverse returned"? That would be the best way to balance flow, or at least as a starting point.

many thanks for tip - I have been looking at the SV to at least sort out the air problems.
For a poor ignorant DIY'er - what do you mean by "reverse returned"?
Do you think the SV would have any effect on the apparent lack of heat getting to the last rads in the system? Or might I have other probs?

Actually, after I posted I thought that RR might have tied you up. Draw 5 boxes side by side. Let's assume you are feeding water to them (supplying) from the lower rt side. Number them sequentially R-L at the lower rt hand corner.

Now the return side. What you want is to balance the flow as closely as possible without using balancing valve. So, let's assume you pull the return off the top rt on the same side you supplied. Number those at the top, again, sequentially at the top rt corner from R-L. If you then add the numbers in each box together you will have very low numbers in the Rt side box and larger as you go to the L. These numbers represent pressure drop. All the flow will take the path of least resistance and the L side boxes will be starved.

What you are trying to do is end up with a total of 6 in each box, representing equal pressure drop in each box. The way this is stated in hydronic land is "first on, last off". So with the same boxes let do the outlet number at the upper L corner from 1-5. The last box on the L will be 1 & that on the rt 5, totaling 6 in each box. Pipe it that way and you should have flow to each zone. You will have to balance various zones depending upon the pressure drop at each emitter, but at least your mains won't be fighting you too. Did I further confuse you here?

Many thanks for your comments. Your explanation of "reverse returning" sounded complicated at first, but then the "penny dropped" and it began to make sense!
Forgive me for stating what might be obvious, but it would appear that "reverse returning" is the correct configuration.
From my schematic (which is pretty much how it is physically piped-in), my system is certainly not "reverse returned" and this might explain my inability to successfully
balance the radiators. I have ended up "choking off" the "close in" rads (using the lockshield valves) to give the "distant" rads a chance of some flow, that the whole system may be struggling (pump and boiler) to work efficiently
I'll have a word with my installer!
Regards
Alan Dummer