I Prefer the Magnaclense and they need to be on the return close to the boiler

Is it an open vented system with a cylinder
?

You could have a problem where the cold feed and the vent meet, a pic showing the pipes from the pump to the header tank would help

Yes, open vented with a copper cylinder and 11 rads.

Can't really get a photo of most of the pipework as it is helpfully under the floor.  From memory, the vent is in 22mm and is on the flow pipe from the boiler about a foot before it gets to the pump.  The cold feed is in 15mm and joins the return coming from the cylinder coil.   The pump (previously the original danfoss ups 15-50 fitted in around 1990 and now a danfoss ups3 15-50/65) was run on the slowest speed

I don't think the system was particularly well laid out when it was installed.  There's a problem with reverse flow round the top floor radiators when only the hot water is on.  Although this usually isn't a problem, as we have a wood stove that provides most of our heat (it doesn't heat water for rads, but a couple of small PC fans in the corner of doorframes move the air around to nicely heat most of the house) so I just turn the rad valves off.  

I think in an ideal world, the entire system would be ripped out and start again from scratch, but being realistic, it isn't going to happen.  

But as the boiler is going to need moving / replacing later this year, it would seem like the sensible time to get the plumber to fit a magnaclean for me, particularly if it wants to go next to the boiler

No, the header doesn't get warm.

It does need some modifications though.   I think the problem is the layout of the return pipework, although I did wonder if it might be easier to sort by adding some injector tees to stop the reverse flow.  Although as the heating is so rarely used, I'm more inclined to just keep the rads turned off which gets round the problem for now

With the age of the existing boiler, it would be a sooner than later exercise, perhaps on the back wall in the new garage?

Look at Intergas as a replacement and if you haven't got an engineer look on their website for a platinum installer in your area, or ask here.

They ain't a patch on Intergas they do a vertical flue which can be used as a sealed system or with the existing open system