Dave wrote:

I suppose, to cut the story
short, I'll be losing nine rads-worth of water and replacing it again over a
period of about three months in total, maybe slightly longer.

What should I do about inhibitor? When should I add it into the system? How
much should I add in, and where? I've got a couple of 5ml syringes that I
could use to squirt the inhibitor into the rads through the hole where the
bleed valve goes after reconnecting them and before refilling them - would
this be OK?

If it were me, I'd leave it until the 2 weeks before the last rad was
meant to come off, then put a dose of system cleaner in.
When you take off the rad, flush the system out.
Refit rad, then put inhibitor in.
If you only have tiny bleed screw inlets to the rad,then a 50ml syringe
will work fine, I've done it a few times, takes 10mins or so, but does
it without spilling any.
Alternatively, inhibitor is available in sealant tube type tubes, so can
be injected in with the usual sealant gun.
If you have a header tank, it could go in there if you are filling a few
rads at one time, though I tend to inject into a rad, as you know it
will get all around the sytem then. Put the last 50ml or so into the
header tank, then some of that solution should get into the feed pipe
going to the boiler when you bleed the rads.

Alan.
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