My advice to you Andy would be to hold on to the bike, selling it and then regretting it afterwards will be a frustrating and possibly expensive experience. As others have said, you can perform a basic bike fit by following online guides. I would also measure your bike to check that it is in fact the correct size for you.
Also try a regiment of cycling specific stretches and exercises, they will not take long and they could yeild results fairly quickly.
If both of these options fail then taking the bike to a bike fitter would be a good idea, even if you have to save for it. They will be able to tell you if the bike can fit you and get it as close as possible.