If you're having severe muscle problems, pain, and cognitive problems, that effectively rules out Oxford CFS and very strongly implicates a physical illness. So LP still won't help. If you're really not sure and have some cash to spend, I'd highly recommend seeing a real ME/CFS expert. They can give an opinion about your diagnosis, and they're going to be a lot better at differential diagnosis than the typical NHS "chronic fatigue" stooge. A first appointment with Dr de Meirleir is something like 60 or 80 euros. Testing is much more expensive (around 3,000 euros) but certainly not obligatory and could help quite a bit in finding a proper diagnosis. He also does a 1 day CPET, which is a very good way to document definite disability. A 2 day CPET might be something you could request, if you want a definitive answer as to whether or not you have ME with PEM. I think that one was about 100 euros for one test.