Are you sure it needs replacing? That would be an awfully expensive exercise. A Gledhill cylinder is made of stainless steel and should last for decades, especially an electric one with no heat exchangers to furr up. However any unvented cylinder does need regular servicing for safety's sake if nothing else. The things that might cause problems are the heaters, thermostats, pressure relief valves or expansion vessels. All thse can be replaced or in the case of the expansion tank re charged in the case of a Gledhill.
For a job like this I'd call up Pimlico Plumbers or someone of that ilk. Not the cheapest, but whoever services it needs to be NICEIC certified, which is completely different to the sort of CORGI/Gas Safe registration needed for boilers. WIth a pressure cooker in the airing cuboard whatever is done needs to be done right in case it explodes!
I have just replaced a Gledhill boiler in surrey keys and the owner is over the moon with the price and service - he'd be happy to let you see the finished product (i even remodelled the shelving in the cupboard to maximize space at no extra charge because i take pride in my jobs).
I design power stations for a living and this is my "hobby" so I am extremely competitive: it may help you or not, but I can only do these jobs at weekends.
All boilers including these can corrode, not all are stainless steel. Happy to show you the job i've done and inspect yours for free, when i can most likely assess if it needs replacing or can be serviced as borouhbloke rightly suggested (thermostats, leaky joints, heating elements etc) (I need to accumulate the first good reviews / get known in the area!) and quote you.
If interestd please do call me!
07766 057 847