How can I fix my Potterton Profile 60e boiler?

My Potterton Profile 60e seems to struggle to fire up whenever it needs to kick in - that is either at first start up or even when it's warmed up, when it stopped as it reached the required temp and then needed to kick in again when the temp dropped. It makes the right noise as if it is going to kick in and then doesn't and tries again and again until it manages to fire up. Sometimes this can be after 15 attempts. Any ideas?