Scott Mills wrote
Ahh, now I have re-read your post and looked at the picture it has all
become clear. I would build a wall next to it as you suggest ie on the
side away from the camera. Use a few large breeze blocks so that the job is
quick and cheap - remember to put in a DPC. You don't have to be neat on
with the cement either, and it might be worth treating the wood too while
you're there.

Scott

OK, but remember the settlement is likely to be due to shrinking clay (North
London) so if you build it straight off the oversite it will probably settle
(relative to the main house walls) again in time. Ideally it needs a decent
foundation, to the same depth as the main walls. Also, make sure you leave some
holes in the wall to allow air to circulate.

Peter