A chipped ECU is the proper way to compensate for extra boost with correctly modded fuel maps.
An FMU is not a "Fuel Management Unit" as the name would imply, it's an adjustable pressure regulator. It increases fuel pressure with boost, making the injectors flow a little more. It's a very old and very crude method of budget tuning, and is limited in tune-ability and overall range.
An "FMU" won't really be necessary on a Capri unless you've added larger injectors & using the FPR to dial them back to stock flowrate for off-boost use
