Baxi Bermuda Boiler Problem?

We have a wonderful Baxi which up until last night was heating our 3 bed house great.
However it has now gone off and we are not sure how to proceed.
The gas is working and pilot light is on, we can use the gas fire manually, but the electric ignition is not working (ignited by hand).
We have replaced the fuse twice and heard a popping noise from within the boiler, and the fuse blew immediately.
Has anyone heard of this and is it a major fault or simple to fix?
As this is an old boiler (fitted 30/1/75) we are wondering whether it is worth fixing or opting for a full overhaul with a new combi?
Can anyone offer any advice?
Many thanks in advance