i'm just heading off for a checkup from the doctor (an MOT as we call it in
the UK) - a good opportunity to double check that being veggy for 30 years
and vegan for 8 has not had any undesirable effects on my body.
Does anybody know of any specific tests that i could request. I presume
iron and cholesterol levels are routinely covered, but can tests be done for
B12 or omega 3 etc.?

B12 is quite checkable.
If you're not supplementing or not a sunshine fiend, worry a bit about D as
well. They can check that. If you're concerned, get a 25-OH D level.

Low D can spell an osseous disaster.

I don't think omega 3's are readily checkable.