I don't know if anyone has come across this before but I just drove the family Roomster & the heater was up full at the 28 degrees setting, so I turned down to in between 22 & 28 & it felt quite cool almost cold

There seemed to be a big difference in temperature from being up at the highest setting &

turned down a little. Basically I'm not sure if there is a fault or not & was wondering if anyone could enlighten me.

It may well be fine, being semi-automatic, depending on the interior temperature.  We normally have ours set between the nine and ten o'clock dial postion, which is probably approx 18/19 degrees C.  From cold it will get quite warm for a bit, then cool as it detects the interior temp rising to target setting.  I prefer a manual setting control myself, as the Roomster control can be a bit lax about providing fine tuning.