Roger Mills wrote:
You can buy energy monitors which either use a clamp sensor round one of
the meter tails or count flashes on new-type meters. But they only
appear to give instantaneous readings and daily totals.

Does anyone know of anything more granular - preferable with the ability
to capture the data and insert it into a spreadsheet? Sounds maybe like
a Raspberry Pi application?

https://openenergymonitor.org/
does it on a Pi, and will sell you the parts need to hook up to your meter
for not-very-much.

If I were to have a smart meter fitted (which I've hitherto resisted!)
but to stay on a fixed tariff, could this be made to provide me with the
required data?

Yes:
https://guide.openenergymonitor.org/.../octopusagile/

Theo