What are the odds on the actual immersion heater elements inside the
hot water cylinder being knackered & tripping the circuit?Is there
anyway of knowing that the elements are knackered from just looking at
them?If they are the problem which I suspect, are they easy to remove &
replace?

Sounds like this is the problem - can you disconnect the heater's Neutral
only, then see if it still trips? (you need to tape up the this disconnected
wire, and not touch anything in there when it is on, obviously)

If this still causes tripping, disconnect it completely, and try again, if
this still results in a trip, then it isn't the element, but if then stops
the tripping, pretty sure it is! (How old is it?)

When you say it trips, do you mean an RCD or MCB (You mention a fuse in your
post too)

Sparks...