Steve, it was suffering from a back problem, showing itself in the form of sciatica, that was the reason I came to cycling from running. I too had a nerve block injection which took a little time to work (several weeks), but improved things massively. However, after 6 months it wore off and the inflammation around the nerve returned. As there are so many causes or more accurately, variations of the same cause, I would also recommend you get it checked out by an expert. I paid privately for a MRI scan (after a physio assessed me and identified which part of the spine was causing the problem) to cut out the NHS waiting time of 3 months +, then took the results with me to the consultant. The solution may just be to reduce the inflammation that is causing the irritation to the nerve, or may require other intervention. Either way, get an accurate diagnosis, and then look at what your options are. But if the cycling doesn't make it worse, keep going! Good luck.