Bearing in mind the cylinder/heater have been in place for 25 years how
difficult will the heater be to change.

There's no need to take the old one out. Just put an additional one in near
the bottom, which just requires a hole saw. This is a much safer alternative
to attempting to release a 25 year old one. Also, check that the old one is
indeed broken. It could be just a broken thermostat, which is easily
replaced.

If you do attempt to remove the old one, loosen it with the tank full.
Rather than putting on lots of torque to the spanner, make short sharp blows
on the end with a hammer. The shocks will be more effective at loosening it,
whilst reducing the tendency for the cylinder to distort or crack. However,
you should note that the chances of failure are quite high and should have
an alternative solution in mind (i.e. cylinder replacement) and the ability
to rapidly drain the cylinder without causing damage to floors and ceilings.

Christian.