How can I fix my lockout problem?

I have a heatslave 20/25 that has been working okay, though ever since we moved into the house and acquired it it has never done a good job of heating the water long enough to run a bath. I had it serviced recently and he said it was old and needed replacing, but to be honest I didn't believe him - he said it was 20 years old when in fact I have the receipt and it was installed in 1999 so it's only 10 - is that old for one of these?

Anyway my problem is that it locked out today and despite pressing the burner reset button I can't get it to start up. Anyone have any advice?