The NICE guidelines also state:
I would not follow NICE guidelines for ME/CFS treatment.
@tinacarroll27 - There is evidence that B12 can help some people with ME/CFS:
http://www.drmyhill.co.uk/wiki/B12_-_rationale_for_using_vitamin_B12_in_CFS
https://www.ncbi.nlm.nih.gov/pubmed/25902009
And no, all the tests don't always come back normal. Often people with ME/CFS have high B12 on blood work because it is not being properly used by the cells. So the numbers can be misleading.
It is good your doctors are doing more checking. Large red blood cells are often an indication of B12 and/or folate deficiency. My MCV (mean corpuscular volume) number was high-normal, near the top of the normal range, and I found that when I started taking folate (had already been taking B12 for years), my energy markedly increased. I also discovered I had to start taking potassium as well. If you start B12 and/or folate, this can cause potassium levels to tank because your potassium need can increase as cells start to divide and work as they are supposed to. There are many threads on this board about B12 and folate, and potassium.
Injections can be effective, if you do enough of them. I used to do B12 injections 3 times a week and noticed no difference. It wasn't until I started taking liquid methylcobalamin (5000 mcg.) 2 to 3 times a day that I noticed a difference. Also, if you take folate you have to take B12 as well because folate can mask a B12 deficiency.