I managed to get hold of an adapter for my Hawkeye Total (not so "Total" apparently)
The results are as follows:

1. 5E17 CAN Gear Plausibility
2. 5DF4 Voltage low
3. 5E5D BLS Electrical Fault
4. Unknown fault
5. 5DF5 Internal Fault

Ok, so I didn't expect it to say "Ah, you have a faulty sensor. I'll just eject it and order you a new one online. Would you like me to make a cup of tea while you wait?" (although they probably could these days)
But what does that mean?

I think I'll just clear the DTCs in case some of those are old, and see if another one appears.

There are a couple of relevant codes here.
Gear plausibility, means an issue with a switch, either the reverse switch, or the first gear switch.
Basically you can't have the box in 1st and reverse at the same time, so it's flagging a fault.

The BLS is showing an issue with the brake light switch, which could have a faulty contract.

Ignore the low voltage code, that's common.

The intermittent code doesn't seem to have a description.

Nodge68 said:

Gear plausibility, means an issue with a switch, either the reverse switch, or the first gear switch.
Basically you can't have the box in 1st and reverse at the same time, so it's flagging a fault.

Click to expand...

Never replaced those before. Is it relatively easy? Not gearbox off I hope

Nodge68 said:

The BLS is showing an issue with the brake light switch, which could have a faulty contract.

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Hmm I'm pretty sure I replaced that switch some time ago, but maybe I'm mis-remembering - I have had a few FLs now.

Nodge68 said:

Ignore the low voltage code, that's common.

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Awesome

bukko said:

Never replaced those before. Is it relatively easy? Not gearbox off I hope

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No need to remove the box to change the gear position switches, but depending on the box, the switches are in different places. It's easy to test them though, so no need to change them randomly and hope.

bukko said:

Hmm I'm pretty sure I replaced that switch some time ago, but maybe I'm mis-remembering - I have had a few FLs now.

Click to expand...

There are 2 sets of contacts in the BLS, one will set of contacts will close when the pedal is pressed, the other set of contacts will open. It's designed as a failsafe system, so there are both opening and closing contacts, working in tandom , but opposite each other.

Nodge68 said:

There are 2 sets of contacts in the BLS, one will set of contacts will close when the pedal is pressed, the other set of contacts will open. It's designed as a failsafe system, so there are both opening and closing contacts, working in tandom , but opposite each other.

Click to expand...

That seems like the easiest option and probably wouldn't hurt to replace it anyway so I'll start there.
Thanks (again)