Even if there are no nerves impinged in the back this does not mean a nerve is not being impinged further down the leg. Have you experienced any foot drop or weakness in the leg? If your common fibular nerve is being compressed at the level of the fibular head, for example, this could cause burning pain and/or numbness downstream in the calf or foot. This nerve can become compressed or injured from crossing one's legs too much or wearing high boots. Similarly, the tibial nerve going into the leg sends branches to the calf behind the knee and also into the foot. Certain activities such as running, hiking and weight-lifting can cause the muscles of the legs to hypertrophy and press upon the nerves. I would suggest you get nerve conduction testing (NCV/EMG) to look at the integrity of all the nerves from the back down to the foot to see which ones are affected and at what location.
A pinched nerve should show up on MRI. While this provides detailed anatomic information, however, it does not provide any functional data (i.e how the nerves are working or malfunctioning). You would need NCV testing to look for this. In the meantime, gabapentin is an excellent nerve stabilizer and pain control agent. Most people that will benefit from it do so at 600 TID, but because there is a "threshold effect" for some, you might need to hit the higher level before noticing any improvement. You can use this agent in addition to non-steroidal anti-inflammatories or even a course of prednisone, both of which reduce inflammation and can take pressure off a compressed nerve.
Peripheral nerves can repair and regenerate quite readily once the cause of the damage is determined and ameliorated. That said, it tends to do so at its own pace, about 1-3mm per day. Only when the nerve has been severely injured for a prolonged period or actually severed does it fail to improve. If your symptoms have been stable or improving slowly, this should not be a major concern. If they are worsening, you should press upon your GP for a sooner appointment with neurology or get NCV testing, which will give him the information he needs to make an informed judgement about this.
The nerve should heal proximally first. If the pain has actually moved down, this might indicate a different type of problem such as a muscle or deep tissue injury, or a vascular issue. Getting an MRI of the leg (or a CT, or possibly an ultrasound) would help to sort this out.
Does this make sense? Do you have any follow up questions about this?
There is a good chance this is sciatica, an irritation of the sciatic nerve which runs from the spine, through the buttock region and down into the calf and beyond. Pain can persist or recur for many months or years and yet very often nothing shows up on MRI. Physiotherapy is an excellent idea as they are almost always able to help. They can also help you get into neurology sooner if they detect something on exam that is worrisome to them. Surgery is not likely to be necessary in the presence of a normal MRI of the lumbar spine. However, you might need further imaging studies such as a CT or MRI of the pelvic region if thing continue to progress and an answer is not forthcoming from your healthcare providers.