It could be this http://www.drfoot.co.uk/heel%20pain.htm
Or it could be the onset of arthritis.I have pain in my heels from osteoarthritis where a spur has grown on the bone.
Yes that's the one Jules - a couple of years ago, my husband had those very painful symptoms too. He had to have casts made, then small inserts for his shoes. Thankfully, since wearing these, the pains have subsided.
Have you been to see a chiropractor or podiatry specialist? (your doctor should be able to refer you)
I am flat footed and suffer from various problems, including heels spurs. I now have hard plastic insoles for my shoes to make me walk properly and it does help with all the aches and pains.