Christian, I don't think I fully understood your point yesterday but
having spent a little time I now think I understand.

The idea of having hot water at mains pressure does appeal, especially
as I am planning on installing two new showers in the house over the
next few months, however I had never intended replacing the existing
cylinder and I am being forced into considering my options due to an
apparent leak.

The point is, cost is an issue for me as is speed of installation, I
am hoping to be able to do everything over the coming weekend and
still have hot water by Sunday evening (and without missing the
England game). The option of moving everything up into the loft is
something I had been considering as part of my future plans to help
increase space in the smallest bedroom. I am fairly keen that the
existing central heating system is still used to actually heat the hot
water regardless of the type of cylinder it is stored in.

If I was to choose an indirect combination cylinder as I originally
planned then I believe the job would just involve redirecting some of
the existing pipework. Is there any additional installation required
if I go for the heat-bank system as suggested by Christian ? It isn't
clear to me whether there is additional wiring needed for this.

Also can anyone give me comparitive costs, my existing HW cylinder is
one of the smaller ones, I believe it holds around 90 litres. I was
planning on replacing it with something of a similar or slightly
larger size.