Merrida, thanks for replying to me so quickly! I'll answer your questions as best I can - they've ruled out phlebitis, the ultrasonographer felt that they were "probably" soft tissue swellings, and seemed to think they had probably always been there - couldn't explain why they had suddenly started hurting though!
The pain itself is quite severe when anything hits my shin (which doesn't help with two small children constantly kicking them!) - it has reduced me to tears before now after just a mild knock. Recently, I've also started to have shooting pains down both shins, mainly when my dog is dragging me along on a walk, and when driving, but occasionally for no apparent reason.
The lumps are bilateral; although not equal in both legs, I seem to have only 1 in the left leg, but quite a few in the right, and there are no other lumps anywhere else on my body.
As for a biopsy or an MRI, well, the NHS being what it is, you have to fight to get anything they consider to be expensive, and if they do refer you for further investigation it usually takes months for an appointment, so to be honest, I haven't followed up on the ultrasound appointment yet.
As for what they think is causing the pain, my GP just said it was neuralgia (hence offering the Amitriptyline to deaden the nerves), but they don't seem to know what the cause is. I know I should go back and try to get more help, but I am sure that they would be unwilling to do any further investigation, and seem to be treating it as though it's just some little deformity that I should just ignore, hence the reason I was trying to find out any information I could before going back. I'm sorry if I didn't really provide enough information before, it's hard to know what to post, so I just kept it as basic as possible.