Managed to spend an hour on it today.
After trying various things I pulled the alarm relay out and managed to crank the motor but as the fuel pump wasn’t running it obviously didn’t fire up.
Went back to basics and although I’d previously checked all fuses more than once, decided to recheck and sure enough fuse six had blown preventing the engine from starting.
No idea when this would have failed.
Plugged the BBUS back in and still have the original problem so decided to leave it off.
I then had to re-sync the key-fobs yet again I’m getting good at that!!
So now everything works apart from the horn and according to my manual the horn fuse also protects the interior lights but they are working so I’m assuming the horn is wired through the BBUS as referred to by JimS
I presume I just need to put a live feed to the horn and we’re good to go.
At least I’m getting good at resyncing the keys.
When I have more time I hope to investigate more, paying particular attention to the reply from JimS

Thanks to all for your input.