I had this a few months back. Had me hobbling around wincing especially first thing in the morning. Finally I droped into our local NHS Wallk-in clinic (hobble in in my case). The nurse asked a few questions, then told nme me what it was, and gave me a handout about it with a coupel of exercises. She was clearly very pleased to have something that could be sorted out without any need for medicines and that.

The exercises were the one katklaughing mentioned, where you lean forward against a wall, and one where you keep your heel on the floor while sitting down, and raise your toes. Worked like a charm.

That one patty clink mentioned - "Loop a stretch band or buckled belt around the ball area of one foot." An bike inner tube is excellent for that - I had to do that kind of exercise when I was gettingover a knee op a few years back.