If so and based on your experience what is the likely hood of being able to remove and replace the immersion heater element shown below without destroying the cylinder? Looks like it has been leaking for a while.

I’d be more worried about the outer flange to body joint – it looks a little corroded there, and they do break easily once they are forced a little.

I’d be more worried about the outer flange to body join

Yes. It has corroded there too. I was hoping that the outer casing was just containing the sandwiched foam insulation.

Is it copper or stainless?
If the copper has corroded around the flange, I’d be budgeting for a new tank. They are not too expensive, £100ish, fitting takes 3 hours typically.Oh, and get stainless to replace copper, longer life, cheaper.Make sure you have the correct immersion too – normal copper ones should not be used in stainless tanks.
In the meantime, you may get some joy using LSX sealant around the leak. It’s only a short term fix, sometime it does work.

Try cracking the immersion heater with the cylinder full of water as it helps support the cylinder.