Afternoon all, rarely post but thought this might help someone in same situation.
Low B12 has many causes, in my case PA was suspected but never confirmed, instead of ever finding root cause injections were given and a "goodbye" after.
I have since found out via own experimentation that:
Antibiotics kills stomach bacteria - I have had many courses in recent years for different things.
Not stomach bacteria = no absorption of key vitamins etc leading to low B12 and other things.
After flooding gut with probiotic drinks and tablets it can be turned around in a month.
If taking any antibiotics also take probiotics - shame doctors never told me this, years wasted.