I repaired a leaking caravan boiler a few weeks ago, I was very surprised that parts are still available. The one I did is only run off electric although it is a dual-energy boiler.
What make/model is the boiler?
Sounds like the part may be the pressure relief valve (or could be called pressure vent valve) which was definitely available for the 1994 Carver Cascade boiler I was fixing and less than £15.
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