Thank you everyone who gave me advice on writing to my GP. I wrote a firm, detailed but non confrontational letter and he phoned me up and agreed to let me have 12 vials, syringes and needles and will review my treatment in 6 months. He wants me to gradually space my b12 injections out to one a month then later on to every two months. I haven't told him I’m injecting weekly with my own supplies as yet, just told him I’ll try spacing them out.
As he had been insisting on 3 monthly injections I think it’s quite a good result. I’m still waiting to see the neurologist but it doesn’t look as if it’ll be any time soon.