Andy Hall wrote:

For heating running for a third of the day, the implication would be
of 5.5kWh per day. However, it's unlikely that all of the heat could
be recovered - perhaps 30% - so we are talking about say 2kWh/day.

Why only 30%? Genuine question. If you run gases in opposite directions
you always have a thermal gradient between incoming and outgoing gas which
makes countercurrent heat exchangers potentially very effective.

This works out to about £15/year.

Unless more than 30% is attainable.

Of course, this is estimation, but I think it's reasonable
approximation within half an order of magnitude

There are other places to obtain a better ROI.

Not if you can knock one up for £15. ;-) Bit of plastic drain pipe and a SS
exhaust cutoff from the local Quikfit.

Tim