I've seen this question asked before but apologies, I just can't think what to search for..

Here goes... for SVA I understand two locking/immobilizer devices are required.

I am using the standard Sierra lock and key. Does that count as two? One for the switch and one for the steering lock??

It has an autojack immobilizer fitted but the wiring is absolute atrocious so I'm tempted to take it off.

Yes, it is all you need.
I personally do not like steering locks so I have an Fia Keyswitch plus a transponder immobiliser. The key and transponder fob are encased in resin to make it sva friendly.

Description

Ignition switch and steering lock count as 2 separate items. Its only if you remove the steering column switch gear (such as i have done) that you need a seperate isolator switch/immobiliser.

David