Jukes

Hi All, I am a complete novice after a little bit of advice from one of you experts please? I have just had a new shower fitted in my ensuite upstairs and wanted some advice on any simple, cheap ways to improve the water performance? Brief details: gravity fed system thermostatically controlled shower valve with 200mm square shower head (ceiling mounted) shower pump 1.5 bar twin impellor (positioned in loft) If I turn the heat dial on the shower valve to full cold I get a great water flow rate, in the default setting the water isn't very hot and not very good flow-rate. When you turn it onto the hot side it is nice and hot but very weak, almost dribbly. I'm sure the best solution is to replace the boiler as it's a gravity fed system and apparantly not really very good with modern showers? (it's over 10yrs old) but I was wondering if there was any other simple ways to give an increase in the hot water flow-rate? Would changing the fixed shower head for a smaller one, like 125-150mm (current head is 200mm sq and has 100 water nozzles, would a smaller one with say 50-60 nozzles make any difference?) Would increasing the amount of time the boiler heats the water every day make any difference? Thanks in advance.