In a word - No. Don't guess, test. That what I was always taught. Test the wire, if you don't have a multimeter a bulb and two pieces of wire will check if its live or not. To me it looks like and earth. Trace it back, see where it attaches. The handbrake wire should be white/yellow trace.

Your photo shows two wires (both black?) attached to the sender. If your gauge works don't attach anything else. The spare terminal might be for the low fuel warning light. It sounds like you are missing the wire from the dash light to the hand brake.

The green/black trace wire usually is the sender to gauge wire. The second connector on the sender should be the low fuel warning light 'switch' usually a white/grey trace wire IIRC. Black earth. The handbrake wire should be White/Yellow trace. Wiring diagram here: Defender wiring diagrams...

When you turn the ignition on, does the fuel pump run? If not does the temp gauge go into the red (even though its cold?) If so it might be your fuel pump relay is duff or the pins/socket are dirty or loose. Also it might be a bad earth near you ecu.

Agreed, see the points system here: Vehicle registration: Radically altered vehicles - GOV.UK (www.gov.uk) Your vehicle will retain its original registration/status with just an engine change. This will mean historic tax (free) however to be MOT exempt it hasn't have to have been 'radically...

MOD F 654 is the Vehicle Release Form It might be possible to get a copy of it from the MOD by asking for a FoI request like this person did here: Request 654 paperwork for named ex MOD vehicle (publishing.service.gov.uk)

Your VIN matches an MOD Land Rover as per your Heritage Certificate. Even allowing for an error in one of the letters (109 instead of an 88) I would still expect to see a few MOD items on your vehicle. I wonder if it would be possible to get a copy of the MOD release papers (MOD F845 IIRC)...

There was similar dilemma for a EMLRA member a few years ago. IIRC he tried to solve the problem by dealing with the DVLA but it ended up him having to scrap the vehicle then break it for spares. As you say the VIN matches the V5 so where's the problem? Who goes around decoding chassis...

My gut feeling is you own a VIN/chassis plate from an ex MOD 109 and a stolen/written off SWB. Someone for whatever reason has married the two together. If you keep the vehicle and say nothing all will be well, unless it's involved in an accident then the insurance will try anything to not...

Can you find any trace of chassis number on the front off side dumb iron? Does the chassis look like it has been shortened? Does it have a removable (bolted in rather than welded) gearbox cross member?

FoI https://www.gov.uk/government/organisations/ministry-of-defence/about/publication-scheme cio-foi@mod.uk

you can ask for a Freedom of Information request but I don't know if they can do that with just the VIN. I'll see if I can find a link for you later. have you any photos of the inside? From the photo you've posted I can't see any military features on it at all. It might be a CL (civilian...

The DVLA MOT website has it as a SWB (?) https://www.check-mot.service.gov.uk/?_ga=2.95073311.22114310.1628975898-121460054.1610306857

Decoding your VIN here https://www.dingocroft.co.uk/acatalog/vin_numbers.htm confirms it as series 3 109 with a 4 cylinder petrol engine

Agreed, something doesn't add up here.

A rare beast, worth A few bob now I would think. Did I see one recently on Bangers and Cash or Salvage Hunters Classic Cars?

I had a Ninety with the same 2.3 engine in it, the timing chain rattled for about 10 years before I got round to sorting it. I fixed it in an afternoon, had I known it was that easy I would have done it nine years earlier!

If you think it's your diffs overpressuring, check your breather pipes are clear. Is the oil ep80/90? Give it a sniff to be sure. Oil leaks can be swine to trace rarely does it just drop vertically downwards!

Glad you got back ok in the end

I agree with @discool some form of immobiliser. If it's a petrol may have been connected to the coil, if it's a diesel the fuel solenoid on the fip.