Help with VDO gauge - oil temp

Hello,
Have an odd issue with a VDO oil temp gauge that I'm hoping for some help with.

Car is a Mk1 Campaign. The MFA  oil temp works fine - taking the temp from the sensor near the oil filter head as per normal.

When I bought the car about 20 years ago it had three VDO clocks in the centre console:

1. Voltage - works.
2. Oil pressure - works.
3. Oil temp - never worked.

Last few months in lockdown I've tidied up some of the wiring - replacing old wiring and getting rid of scotch clips from the previous owner - only taken 20 years!!

I had a look at the Oil Temp clock and could see 3 wires: +ve, ground and switch. The switch wire ran into engine bay where someone had previously cut it and left it hanging - not connected to anything.

So I bought an oil temp sensor from Heritage that repalces the sumpl plug.

When I connect the the switch to the sensor when stone cold - it shows 50c - MIN. As soon as I heat the sensor up to say 60 with hot water it fires right across to MAX.

I then connected the VDO switch wire to where the MFA gets its input at the oil filter head. Car is stone cold. Start car up and it stays at 50c. After a couple of mins it then flicks round to MAX and stays there. So it's acting like the sensor wire is grounded.

To me this sounds like a faulty gauge - but before I dish out my hard earned and get a new one - any ideas from others. I don't have the original wiring diagram - but the clock is the same style as the oil pressure - whereby there are small spade connectors and looking from above its: GND/SWITCH/LIVE.

Have checked resistance of the new sensor and seems OK. If I hold the sensor in my hand resistance gradually goes down.

Any thoughts?

Cheers!