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Yes, what Metz said but it could also be that the heater matrix is blocked with scale.  It has happened to me after being drained down for the winter.

I backflushed mine with a hospipe held against the drain plug and you can either open the hot taps or, as I did, remove the red pipe from the outlet of the heater, replace with a short length of hose and stick it out of the window.  Amazing how much crud came out!

Iv'e just done one today, not saying this is your problem but it's one more possibility?

Chap and his mate had tried alsorts to get water out the cascade but nothing worked so they sent it to me. Before I started I pumped it up to 30psi to see if it was leaking , no, so I stuck a screwdriver down the cold water inlet to open the non-return valve and release the air but it would not budge??

So I unscrewed the water inlet fitting and as it was stood up on the bench the reason was plain to see? The fitting was a later black plastic replacement and a little of the original white fittings threads were still in the hole below it. The new fitting has a shorter thread so screwed in but the valve was held firmly shut by what remained of the old fitting.

Got to say it's a first for me but you never know?