Thanks for all your suggestions.

Although I do drink a lot of water, I shall try and drink more in the evenings, and will increase my potassium from one to two tablets (taking one in the morning, and one before I go to bed with the mag and calcium).

I will see how I get on with this over the next week or so, and will post here again if it proves to be successful - it may help someone else if they are experiencing the same sort of thing.

If it doesn't help, then I'll certainly think about seeing my doctor in case there's a more serious underlying problem.

Demi