OK here goes: My Y reg Disco TD5 ES has recently shown a fault where the cat symbol lights up & the engine cuts out. This has reset itself when the engine is switched off, so I can start the car again & set off - however when over a certain RPM the light comes back on & the engine cuts out again. Does anyone know anything about this problem please?

sounds like the insides of your cat have become blocked or come adrift. This means your exhaust gasses cannot escape. The harder you floor it the harder it blocks up.

I think you can remove it as it is not required by law on a diesel. The engine will go much better with less back pressure.