Urgent 463 300GD Wiring question

Hi, Roly, thanks for looking. When I bought the car the only non-original aspect was a remote temp gauge had been fitted (and worked fine) as the cluster temp gauge had become u/s. Despite the fact it is a one owner car they had likely baulked at the MB price, the equiv of circa £700.
Anyway they had connected this to the single pin temp sensor and I belive taken power from the fuse board. When I gave the car to George Fraser (Merc specalist London) for a thorough check, new glow plugs and an all fluid change, he sweetly found and fitted a s/h stock transistor? board for behind the temperature dash gauge from a correct cluster. It works perfectly. So he disconnected much of the non-orig wiring but a loop was left (or inserted) linking the two pin sensor nearer the bulkhead with the single pin. I dont know if this is correct. Then some weeks later whilst studying the engine (diesels are new to me), I found a disconnected single pin at the end of a black wire with a purple line (and white markings along its length but unsure if these are MB). George tells me that it doesn't fit the temp pin but I'm not sure. I dont want to blow the temp gauge so am reticent to do anything until I'm sure. Hence my quandry.

Roly thanks. Yes the temp gauge works perfectly. Having had this song and dance with thermostats, I have been testing/comparing them with a cooking thermometer in a saucepan on the hob and those fitted perform and correlate with the temp gauge. It is extraordinary what significant performance differences exist from identical themostats, I've 'borrowed' 1x85 (plus my original) and 2x 80's. I couldn't understand why Jesus's replaced motor now runs at 85degrees dead at all times; he is fortunate to have found a low reading 85 themo or has inadvertently fitted an 80 which I have now done. The problem seems resolved but sensitivity is still apparent with about 10 degees difference between deacceleration/no stress and hill climbing; yet at warm tickover after a run it does rise a bit, so maybe glow plugs do remain on. The glow plug indicator goes out at the correct time on start up. The connection to the 2 pin sensor appears original, it is just the loop wiring from it to the single pin, whilst not green, may be rogue. Not being remotely electrical I shall try to dicipher your kindly attached diagram. Any idea how I can reduce it in size to print? Are there any wizard G knowledgable auto electricians in or around London or in Suffolk? regards Tombo