To explain a little more...
A Megaflow would be a great solution, but a very expensive way of solving your problem JulieMG. Briefly put, it's a way of storing hot water (like you already have), but provides it at mains pressure for your showering pleasure.
To answer your specific questions first:
JulieMG said:
(I think we have a gravity fed system?).
It sounds like you do indeed.
JulieMG said:
Do we go for an electric shower or power shower?
Power. Every time. Electric showers are very cheap to buy and install, but are basically a big kettle hung on the wall, and the flow you get is determined by its electrical power rating and the temperature of the incoming cold mains water. Because of this, in winter they are generally miserable. And in hard water areas, they quickly scale up.
JulieMG said:
I would love to have the power of the power shower with the ability to heat the water - does anything close to this exist?
There is a Triton shower, and there might be others, than combine a pump with instantaneous heating. This takes tank fed cold water. I've never fitted one, but I can imagine that this works fairly well, since the cold water is at a higher temperature than the cold mains.
JulieMG said:
Can you add a pump to boost an electric shower?
Mm, no, not really, not in the way you think. This is because electric showers take the water from the cold mains, and to add a pump would be both illegal and ill-advised. However, you can use the previous type of shower, which pumps the tank-fed cold water AND heats it.
JulieMG said:
How difficult is it for an electrician to fit a power shower? I have a friend whose a great electrician but he's never fitted a power shower.
If he has no plumbing expertise then I doubt that he would manage, but the electrical work is easy for an electrician.
EliteHeat's suggestion of a venturi shower valve is an excellent one. This is a cunning device that takes mains cold and tank fed hot, and uses the fast flow of the cold water to boost the flow of the hot - the resulting mixed water flows at an aggregate, roughly, of the two. They're very good, and the Trevi Boost is, IMHO, the prettier of the two that I know are available.
A more expensive option, but probably a better one, is to install a dedicated shower pump, which would take tank fed cold and tank fed hot supplies. This is the type that's generally known as a "power shower".
If you search the forum you'll find quite a few posts on venturi showers, and a great many on pumped showers.
Yet another option would be to install a pumped mixer valve, which takes tank fed cold and hot and pumps it at the shower valve. The cost of these is about the same as the venturi type, but the installation costs are generally higher because installation requires both an electrician and a plumber, and the electrical work is notifiable under Part P of the Bulding Regulations.