Boiler made between 1983-1990 even if parts are available or not, maybe its time to start looking for a change. I bet there are not many other appliances in the property that are older than 25yrs.
Ah so you can justify the £1-2K installation cost to change it for a modern condensing boiler, that's has loads more expensive to replace components & will be less reliable. All to save a limited amount of gas per year.
Granted a new boiler will have an extended warranty for the manufacturer to wriggle out off at the slightest opportunity.
I think you've been listening too closely to the suits at BG??