I have an un-pressurised thermal store that has a 15mm copper vertical
vent pipe at the top that goes up into the loft and hooks over the Fill
and Expansion tank.
Although it is insulated with foam, I have a hunch that I'm losing some
of the stored heat by convection up the pipe.

How much of the heat is likely to be conducted up the pipe itself as
compared by convection in the water it contains?

I have a couple of options.

I could re-plumb using stainless steel pipe which is much less thermally
conductive. Useful if the loss is conduction through the pipe.

I could instead reduce the bore of the pipe and hence the amount of
water contained in the pipe if the main contribution to the loss is
convection.

Someone has suggested that I put a U bend in the pipe run and that the
downward section of pipe run from the store will stop the convection losses.
This sounds like it might work but I'm not 100% convinced.

What does the team think please