Thanks for the link which I read, Paul. I found it pretty technical, probably because biochemistry always does my head in. It's worse than physical chemistry. However I googled "Vitamin B12 contraindications .gov .edu" which revealed this abstract written in terms even I could understand.
Vitamin B12 in Health and Disease (nih.gov)
Most of the literature relates to B12 deficiency but there appears to be contradictions on toxicity across various publications. Some say none and others report a range of effects. In desperation, I pulled out my very old copy of The Merck Index (7th Edition) which often includes toxicity data, LD50s etc. on almost any chemical you could name. No luck.
This topic reminds me of a friend who was asked to work on the elimination of carp in Australia. Almost all the scientific abstracts were about how to breed them for food rather than get rid of them.
mick