Ok - plan B. If you can get hold of a garden tap (one without the double check valve in it!) and remove one of the bleed plugs from the top of a radiator upstairs, then screw the tap into the radiator, then you will be able to connect a hose to this and a mains tap downstairs to enable you to force water into the pipework this way.
Which involves draining the system enough so that the FE tank is empty, plus a bit more, wipe out this tank and add the chemical before releasing the ball valve into operation again.
Yep - all done. Planning to leave the cleaner in the system for a good few weeks (the bottle said it can stay in the system for up to 4 weeks) to make sure it gets a good clean.
Got a bottle of inhibitor on standby too for when the cleaner comes out. Hopefully that should do the trick.
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