Michael Chare wrote:

Does anyone have any recommedations for Surge Protection Devices which
are suitable for use where they can't be fitted in a modern consumer unit?

you can get an SPDs combined with a mains isolator, e.g

https://www.tlc-direct.co.uk/Products/WYREC2SPD.html

or

https://www.tlc-direct.co.uk/Products/CGSRG1VCU.html

How useful are SPDs where all the local wiring is under ground, but some
of the 11KV upstream of the local transformer is overhead?

pass, probably depends if urban or rural, your supplier might give you
some numbers to plug into the formula

https://www.dses.co.uk/index.php/free-advice/198-the-sy2-d-surge-protector-spd

I was surprised to discover that they are fitted in parallel with a
consumer rather before and in series.