Others have said exactly the same thing but hasn't boiler design moved on in 20 odd years? If you believe what's written, anything over 15 years should be replaced. No wonder I'm confused!!
I've a Potterton Profile 50e which is coming up to 21 years old. It's never had many problems apart from a noisy fan which I've got used to. However, recently it's had starting problems. The fan clicks on but after 5/6 seconds you hear another click and it switches off again. This happens 20/30...