I have previously posted a mod I did to my car which involved fitting a BMW E34/M3 coolant expansion tank. The tank is almost the same but it has the advantage of having a level sensor fitted. Can't recommend more strongly having a coolant level sensor. It saves a lot of grief.
Anyway, to cut a long story short, I was just on the Island4x4 webside ordering yet more bits and low and behold, someone now makes a level sensor which directly replaces the screw cap on the p38 OEM tank. The BMW tank mod is no longer needed, stay with the p38 tank.
Can't go wrong at that price even if it is Britpart.