Can't stop kettling.

Okay, I can answer this myself now after managing to completely silence my boiler.

I decided to take the plunge and put some Fernox DS-40 in the thermal store.

I was wary of doing this as there are many forum reports by plumbers of this stuff opening up pinholes in radiators and removing scale that is sealing joints, leading to hidden leaks.

As a result, I decided to keep the stuff in the store and not circulate it around the radiators.

I turned the boiler, heating demand, pump isolating valves and header tank supply off.

I drained about 10 to 15 litres from the thermal store drain cock so that the expansion pipe was completely empty.

I removed the right-angle compression joint from the expansion pipe where it joins the top of the store. I then poured in about 3/4 of the diluted DS-40 mixture directly into the top of the store and put the remaining 1/4 in the header tank.

I refitted the compression joint and refilled the sytem from the header tank.

When it was full, I turned the boiler back on and let it heat up to temperature. Immediately, there were none of the usual noises.

I left it like this for a couple of hours and ran hot water every 20 minutes or so to make the burner kick in and 'stir' the store contents.

After this time, I drained a little of the water from the store and saw that it had turned from red to yellow, indicating that the active ingredient in the DS-40 was depleted.

I then added the neutraliser in exactly the same way as the DS-40 mixture and ran the boiler for 30 minutes or so.

After this I drained and refilled the store twice until the water was pretty clean.

I also added three lots of Fernox F1 on the final fill via the header tank. This should prevent the scale build up in future.

If you are going to try this, make sure that you have got rid of all of the sludge in the radiators first by using something less vicious such as Sentinel X800 or Fernox DS-3 and flushing the system, otherwise the crud will just get pumped back into the store.

Don't forget to reopen the pump isolating valves when you have finished!