Many thanks for you help guys. Its obvious that something is not quite right. I think I'm going play it safe and get an electrician to look at it with the hope it wont create too much patching and plastering work. I have 3 young children so my priority is to make sure its safe.
There is another switch next to this switch which controls the downstairs light. I have attached a pic. When I took this off some of the wires just popped out of the terminal. I have put them back in and still have the same problem.
I have just moved in to my new house and found there is an issue with the landing light. The switch upstairs only works when it is switched on downstairs. If you turn the downstairs switch off then the upstairs light does not work at all. Downstairs I have 2 separate switches units next to each...