Thanks cbell, I think you have cured my insomnia!!!
Your system is dangerous & will not comply with British building regs or water bye-laws!!
How is it dangerous? It's an open vented system, so it can't build up any pressure and explode, and it has a mechanism for dumping excess heat.
And how on earth can it not comply with water bye-laws? There is no direct connection between the CH system and the domestic hot or cold water. The system is filled from a header tank with ball-cock.
Sorry about the insomnia though, plenty of sheep round here if you want to come and count them ...