My TD5 seems to be randomly altering it's revs by about 100 rpm. If I set it at 2000rpm and wait; now and then it will bump up to 2100 for a few seconds and then drop again. It's instant and very obvious both up and down.

Has anyone seen this before?

You mean if you are stationary in neutral holding the RPM at 2,000 it fluctuates.
Or when you are driving along holding a steady throttle it changes RPM on its own?

The TD5 has an electronic throttle pedal and it's known to sometimes give trouble. You could try a new (or good used) one. Also check the red plug on the ECU (under the drivers seat if the Defender is RHD) to see if it's got oil in it. lf there is oil in the plug it can give various issues, and to cure it you'll need to change the injector harness.

lt's a fairly easy job you can do yourself and the part is around £50.

It’s more noticeable when stationary and holding the throttle but I do get the sensation when driving it that there’s a very slight (you wouldn’t notice it if you weren’t looking for it) surge in power now and then. I just assumed it was normal. But stationary, it’s very noticeable and instant, like someone is throwing a switch.

I don’t think there’s oil in the plug. We’re in there most days for one reason or another and it’s only just been fully rebuilt.

The throttle pedal may well be old. The only thing that I know about it is that it wasn’t coded correctly for the ECU. It is now but that doesn’t mean it’s not faulty. Are there any ways to check or is it better to just assume the worst and swap it? It’s probably the only thing I haven’t replaced.

Update: with the MAF unplugged it doesn’t surge. With it plugged in, it does.

It’s a new genuine MAF sensor.