Water temp gauge
 

When fitting a water temp gauge can I just tap into the original temp sensor wiring?

No mate, that sender is only for the OE gauge.
And bear in mind there are TWO water temperature senders, one with only one wire for the gauge, and one with two for the ecu. it's the gauge one you can do without.

You have two options:

Do away with the OE gauge and screw the new sender in where the old one was, or,

Fit a new sender into a fitting like this: http://www.ebay.co.uk/itm/38mm-BLUE-...item1e7c7b38c5.

I first off did the former, but seeing the gauge on the clocks constantly at zero did my head in, so I now have the latter.

You need to make sure you get the correct diameter fitting for your top hose and about an inch of the top hose needs cutting out. I'd suggest a new top hose at the same time.
Also get the correct tapping for your sender, most are 1/8" npt.

DO get a fitting with a screw terminal on it to allow it to be earthed to the car's electrical earth, or your sender likely won't work.