Fitting a New Immersion Heater

Perhaps you'd quote an approximate cost for this against the DIY 10 quid or so for a new immersion?

Of course, I forgot there were money trees in planet Drivel.

No. That is for the OP to take up and run with. I gave the best advice here and gave some products too, one being very cheap.

And also the cost of building the appropriate blast walls as according to my local "plumbing merchant counter assistant" these hot water tanks explode all the time.

Funny the way your advice always involves spending vast quantities of money regardless. Of course you were probably on commission when working, so that explains it.

Also, unless things go wrong, changing an immersion is pretty straightforward. Changing to a different make of cylinder is likely to involve some quite complex plumbing. Which many would leave to a plumber.

Which ones is he talking about?

In the words of Marshall Mathers, part time corgi trainer and purveyor of plastic pipe and copper cylinders to the masses:

"and in the car they found a tape, but they didn't say who it was to Come to think about, his name was.. it was you Damn!"