Perhaps I am baking them at too low a heat due to the current climate in the UK. I will see what happens. Thank you!

I would say yvonne is on to something. This is actually a very common problem in cake baking. A slow oven is always known for causing falling in the middle. Too low of a heat will not allow leveners to work correctly causing poor volume and texture because the cake doesn't set fast enough and may fall. Check the middle see if it has some spring before pulling. Good Luck!

I'm using the same recipes I have done for over a year. The tins are the same too. I have changed baking powder and perhaps my baking soda is old. I will check the heat of my oven with a thermometer as this may be the problem. Thanks guys :-)

I agree with mrslarkin: it could be a number of things; MORE LIKELY ARE: Either you over beat it or you opened the oven door while cooking the cake, before 70 % of the cooking was done. ALWAYS avoid opening the oven while baking a cake.
(not so when baking bread or cookies) Hope I could help.
HAPPY HOLIDAYS !!

Is your oven calibrated? Not knowing the recipe, we can't be of much help here. If it's a recipe you've made before and the batter seemed ok, I'd start w/ checking to see if your oven temp is accurate w/ a thermometer.

The problem could be so many different things. Could be too much leavener. What kind of cakes are you baking? Ingredients? Amounts? Size pan? Temperature?

Could be your recipe?