[Sooty25]

the App I use is "GPS Test"

same here - indeed using that same screen
it started raining, so I stopped for today
I've got a theory to investigate and an app to use, albeit I suspect I might need to drive to some of the drop spots and see what 4G band is in use there

albeit, what I actually do about it in the end I'm not sure!

If it is the 4G transmissions from your phone you might find chucking your phone on the back seat is enough!

[rikur_]

I seem to have a bit of a repeatable pattern on this.
First test was to check the type of coverage used by my phones at the three local GPS 'drop spots' .... in all cases LTE band 3 (1800MHz)
Next test, switch phones to 3G only mode. No drop-outs.
After a week, revert phone to 4G, drop outs resume.

Arguably circumstantial - and I can no longer remember all the coms theory for different interference possibilities.
I was expecting harmonic interference on GPS from new 5G services in the 700MHz range, it's less obvious why 1800MHz band would interfere.