pilot light has gone out, can't re-light it!?

I have a Sime Halstead Trio 30/90 boiler (in house when purchased 5 years ago) It has been fine up until this morning. The pilot light has gone out and I don't know how to get it going again. I have tried switching the heating on, which worked for a couple of minutes and then went out again. I haven't been able to get it lit again.
Can anyone one help?