As above, I’ve always thought the heat of the gas coming out of our boiler flue through the wall is wasted and could be re-used to either heat water in a tank or heat air for the house in winter. Googling shows devices like this: http://www.superhomes.org.uk/resources/passive-flue-gas-heat-recovery-devices/
Seem pretty expensive for the return time. Anything else out there or real world experience of this?

If your boiler is condensing, they won’t gain you much at all – and there will be better value for money efficiency measures.

Pointless. If you haven’t got a condensing boiler, they will save you a bit, but cost more than a condensing boiler, which will save you more.

£1200! Could buy a new condensing boiler for that if you don’t already have one. In fact my CDI 30 cost me less than £1k. If you already have a condensing boiler then as above you are already extracting pretty much as much of the heat as possible. As the gas gets cooler it is diminishing returns for the extra complexity and you start getting poor draw on the flue.

We do have a condensing boiler, from about 2008 I think, flue gas does seem pretty hot to me still. Not interested in spending that kind of money on a device for heating water up as pretty pointless but I thought there must be a (simpler/cheaper) way of capturing the wasted heat for getting warm air back in the house in winter.

Ours is 25+ years old and the flue exhaust is pretty low temp – 78% efficient design from the 1970s!