Unless the heater is really ancient it'll be a metric thread. Measure the (major) diameter of the post. If it's 5mm try M5, 4mm M4., etc. My guess would be M4 but @ShaunCorbs is right about most things and I don't have an example to hand to measure.
Reply to Immersion Cover Nuts in the Central Heating Forum area at PlumbersForums.net
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