Potterton Profile e60

I have a 2 year old potterton boiler. After the heating has been on for an hour or so or after the i run a bath there is a loud single bang from the pipe work near the cylinder or the loft(hard to say exact where noise is coming from, as it seems to echo through the hole house. The bang seems to be getting louder and more violent...any ideas and will it cause any damage to my system. I have a one of thes 12month contracts with Brit Gas, should i get them out ??
Help..Help..Help...!!!!!

COuld it just be a pipe expanding, trapped in the woodwork somewhere?

I do get the noise of the pipes expanding, But this is a single big bang (or thud) much louder than a pipe expanding. Any more ideas Thanx !!!!