I thought I'd better post this as new message as I keep seeing people relying on their GP B12 blood test which is of little us because only 10-30% of serum B12 is actually available to our cells. B12 is vital to our health that it is everyone's interest to get the test above done if at all possible.
It's done at Guys, see link above (other private labs may be catching on now?) cost £18. The contact is email@example.com, you need gp referral letter (my gp just stated I've been exhausted for ages - actually it's years!).
There is quite a bit of info on the internet on how B12 binds to what and why a lot of it is just expelled (in urine) from the body.