I ventured into a DIY store last night and noticed that a "Power Shower" may be the way forward (it's not the same as an electric shower - it has a built in pump). Quote "ideal for gravity fed systems with low pressure". £180 for a good one, probably £80 to fit?
Has anyone had one of these installed? I read that they sometimes need to expand the hot and cold tanks to run correctly which will mean more ££££££
How old is your house/system? - most modern systems have enough capacity...