It does indeed. After the initial drain down and flushing the system out (by that, I mean refilling the system and draining again) refill the system adding the cleaning chemical. Use as per manufacturers instructions (that normally means running your heating at a high temperature, depending on the manufacturer, for at least 2 days) Drain again, flush again to remove any chemicals, especially if you have installed new rads, refill, then add the inhibitor. It sounds complicated, but is quite easy.
No problem 
It sounds pretty straightforward now you’ve explained it further. It looks like it might take at least a day and possibly more, so it might be best if I leave it until the weather’s a bit warmer.
Thanks again. Very helpful.
I’ve just discussed this with Marge and she agrees that it would be best in the long term to do the job properly, draining down the whole system, flushing and replacing any radiators and valves as necessary.
We’ve decided to leave it for now and wait for the weather to improve. We’ve just had some new carpets too, so I’ll have to roll them back first!
Thanks again for your help and advice. 
No problem and if you do decide to change your rads, remember if they are old, they will be the imperial size. Therefore, some new rads will not be the same size(metric) and the pipework might require altering, or extension fittings to your new rads.
Actually, there is only one which definitely needs replacing because it is rusting underneath externally.
Others seem OK at least externally, though the large one in the living room has needed bleeding a couple of times, so I assume it is rusting internally. I may be wrong, but when I last bled it there was a distinctive smell from the gas escaping.
I might try that little trick of collecting some gas in an upturned container and see if it pops!