Complete System Flush

A question about flushing please.

I have got a BG engineer coming out on Monday due to DHW problems which I am asking about on another thread on this site. (Please continue with your excellent advice and suggestions to date!)

Have decided whilst he is there (either during or after his visit), that I will flush the system. I realise that a power flush is not adviseable!

I understand that to flush the inside of the tank I need to connect a hose to the drain on the bottom of the tank, and with the system shut down and F&E float valve isolated, the internal tank can be drained down, flushed with fresh water through the cold fill pipe (as long as its not blocked) and then I intend to add a quantity of Sentinel 400 for a few days before draining again, flushing with fresh and then refilling via float valve in F&E tank and adding some inhibitor. - Is this correct?

Question is, how do I drain , flush and treat the radiators and pipe work part of the system? I have 2 drain down valves on the ground floor radiators so it would seem that getting the dirty water out is not a problem, put how do I put the Sentinel 400 into this part of the system and refill as part of the flushing process?

Or have I misunderstood the system internals and by draining down via the drain valve on the bottom of the Boilermate 3 and then also by draining down at the lowest point of the rad pipes, then by the refilling of the system via the cold fill pipe, this will top up both the thermal store and also the rads and pipework?

Intend to do this on my system and also a friends this weekend so any swift replies would be appreciated!

Many thanks for any advice.

Regards Neil