Next CP12 (sorry, Gas Safe) inspection will be next November, Tony, when the Magnaclean filter will be inspected/cleaned. Didn't know about the water quality check, though it's my plumber/installer who used it, so the onus, as far as I can see, is on him and effectively on the Vaillant agent who registered it. Anyway, Hobson's choice here; sth had to be done, temporary or not.
Third day of monitoring header tank/ingress/leak etc. in conjunction with the flow temp. No leaking when boiler is on and/or the flow temp. is 40 or above (room 'stat at 16). Around 30 to 35, the mains ingress starts to drip (but just a drip; not the flow as previously). This immediately stops when the boiler fires up and flow temp. increases. Obv. still dodgy but this appears to be an improvement. What will happen when we stop using 24/7 htg, I don't know but sooner or later, this'll have to happen. What'll happen when we stop having htg in the spring if there's still a leak, I don't know, as the system will need to be drained down again.
What a palaver ! I'd be tempted to but this Miracle Seal stuff and add to the system but don't want to take any initiative/responsibility for obv. reasons.