James ) gurgled happily, sounding much like they were
saying :

I received a NIP (notice of intended
prosecution) not long ago for doing 32mph and it defines the speed
limit there as 20. I did to be fair see these signs, but always
understood that 30 was the legal minimum speed limit in the UK

You understood wrong.

and that therefore signs indicating 20 were advisory. Is this still true?

It's as true as it ever has been.

Does anyone think I can challenge the NIP or the automatic penalty
offer of a delightful 3 points that will inevitably follow on this
type of ground?

You most certainly can challenge it. You might not get very far. Well,
except court. You'll certainly get there.

Anyone know of cases where people have challenged the
Tower Bridge cameras?

On the grounds that they saw the signs but didn't think they applied to
them, m'lud?

Anyway - I hate to break this to you, but 3230, so you'd still be pleading
guilty to speeding. The 10% +/- 2mph leeway IS "advisory".