There are some unusual aspects to the problems that you report so I need to go back a bit.
The Potterton Suprima range is a conventional heat-only boiler - it is not a combi. The diverter valve thus switches the hot water being supplied by the boiler heat exchange (the primary circuit) between the CH or the DHW feed (or both at the same time).
The boiler switch-off temperature (on the boiler itself) should be set to 75-80. This is the temperature at which the boiler turns off, once the water in the primary circuit has hit 75 degrees. If you now go and set the tank stat at 80, the temperature in the DHW will atempt to get to 80 before it turns off. This is dangerously hot, so the gas fitter was correct in turning that down. It is never necessary to have DHW that hot. Water for a comfortable shower is just around (or over) 40 degrees C, so there is something seriously wrong with your shower set-up if you can't get a decent shower when fed by DHW at close to 80!
Can you confirm that the pressure fed to the cold feed of the shower comes from the same pressure source as the hot? - because I suspect that you have got the cold feed connected to the mains pressure, whilst the hot feed comes from a cold-water tank in the loft? That would explain why the supply to the shower is so lop-sided - but it could be another reason.
(continued)