We've just fitted a new LED on the dashboard for the generator, since my new speedo array doesn't have a light for this (sorry to the purists, pictures attached). I believe this should go out when the engine goes above around 2000rpm?

It doesn't and I think this points to potentially a generator issue or a voltage regulator issue.

The generator sounds like it needs new brushes and a good clean for starters. Beyond that it may be a trip to a specialist auto electrician.

Any thoughts or tips?

To use an LED you will need to install a miniature relay as the LED is polarised
The original lamp had a 12v feed on one side when the ignition was switched - thus illuminated
When engine was started 12v from charging device is connected the the other side of the lamp - and this means the lamp goes out
You cannot do this without a relay with the LED installed

Good piece of info, thanks Andrew.

The dangers of the uninitiated playing with the electrics.