you've got a electric shower that tees of the tank in the loft.why don't you plumb it into the mains water where it should be most electric showers only work right if they have min of 1 bar pressure, gravity will only give you 0.1 bar every three feet from tank. you can put a pump on gravity to give you 1bar, you can't put a pump on mains feed water its illegal. the shower should have it own feed not a tee. also try checking all the shower head and pipe they scale up or kink causing pressure lose. 2.8 kw don't sound right more like 8'5kw.
hi-spec plumbing
October 2006