My case is slightly different as I was actually treated for B12 deficiency in 2006, when my levels were only about 170. I was only ever given the loading dose of 6 injections over 2 weeks, told I was cured and was sent on my way. Unfortunately I was as ignorant as my GP and didn't question this.
Last year I was diagnosed with Graves disease and it was only through my research on this that I realised the link with low B12. In some ways I feel the Graves was masking my B12 symptoms, as it was only when I went slightly hypo and then euthyroid that I realised I was still ill with this. The other interesting thing is that I never had classic hyper symptoms anyway, they always felt more hypo to me. I actually put on loads of weight. I only found out I was hyper by sheer accident in a routine medical.
Anyway, I digress! My serum B12 was around the 300 mark and every doctor I questioned said I was fine, including my Endo, my GP, and an A&E doctor (when I felt so awful I actually went to hospital). My folate was off the bottom of the scale as well, but nobody was bothered about treating this.
I contacted the PA society, who were great and gave me lots of advice and courage. After a very bad encounter with my GP I asked my private Endo to run tests for me which he did. So that's how I found I had elevated MMA and strongly positive parietal cell antibodies. I didn't test positive for intrinsic factor. This is all fairly recent so I haven't been back to the Endo to discuss my results. He wrote to my GP and I went back for B12 shots (I saw a different doc at the surgery)! Despite everything, he wanted to just give me 1 injection and then nothing for 3 months. I waved the BNF guidelines at him and insisted on the loading dose. I'm on 3 monthly jabs now but I'm keeping a diary of symptoms and I'll go back if I feel I need a shot sooner (might not get it). I also take 5mg folic acid (on prescription), a B-complex (Jarrow's B-Right), fish oil, and a methyl B12 sublingual spray (Boost) twice a day. Am thinking of moving on to the Jarrows 5000mcg sublingual tablets.
It's early days but sorting this out has changed my life completely. I feel like a normal person again, not just a shell who barely had the strength to get out of bed in the morning, let alone look after my marriage and my kids.
If you're in the North you should check out B12d.org and Dr Chandy, who knows, might be near you. And I'd definitely seek that referral to the Haemotologist and get your folate checked. Read up as much as you can, as it's harder for them to fob you off that way! I would recommend the books "Could It Be B12?" and "Pernicious Anaemia: The Forgotton Disease".
I have really mixed feelings about self supplementing without a proper diagnosis, as you end up never getting diagnosed because serum B12 goes up quickly even when you're still deficient at tissue level. However I recognise that for a lot of people it's their only option.
I hope this has been helpful and not just a boring monologue