So far I have to recommend electric - with a minimum of 8.5kw. As you have a flat and the water cylinder is at floor level or above, you can't really guarantee that the pressure will be good enough for a power shower - which are brilliant, by the way....there again, if there is plenty of hot water available to fill the bath, you could be ok, but the fact that the shower head is higher than the bath taps could prove an issue. With electric, no problems apart from routing the cable back to the fuse box.
John
April 2009