Leaking boiler. Can I reseal it?

Our boiler is at least 25 years old. It is a Glowworm but unfortunately I do not know the model number. It is is a large floor standing model with five bolted cover plates with 63-8A stamped on them. It has rarely let us down other than replacing the thermocouple and cleaning pilot lights etc. However water is leaking from some of the cover plates. Is it possible to by some kind of sealant or make or buy a gasket? I believe in trying to keep things going where possible. A plumber told me (without seeing it) that it was built to last. I would like it last a bit longer. Would appreciate someone rescuing us. Many thanks. John