On 18/10/2020 21:21, Roge
There is a reasonable '3'
signal at the flat. Any reason why I couldn't use one of these SIMs -
and then top it up or replace when the data is used up?

I tried a long expiry '3' data sim a few years ago, amazingly cheap
compared to the competition. However, the download speed made it
unusable, often in the 300-500 Kb/s range, I only used a small fraction
of the data before I stopped using it. I'm not sure if that was due to
'3' signal strength or contention for bandwidth. I tried it in a few
different locations, miles apart, and it was useless in them all.

So I would do some '3' speed tests, at peak hours, before assuming it is
a solution.

O2 was fine (20+ Mb/s) , but only had monthly data, I would use Giff
Gaff now, which is O2 based.