I'm a little confused (or maybe its you

) - are you connecting a pressure switch to the stock VDO in your centre console???

- this is a temperature guage

there is no reading for oil pressure on a mk1, just an led in the clock cluster that flashes when pressure is low (one of the ones that comes on at start up) and an annoying buzzer that sounds when theres a problem

Have you checked the sender? they're only a couple of quid and do tend to get a bit tempermental with age.
If you can be bothered you could test it (i'd buy a new one anyway) - remove the sender & connect a bulb tester (possitive to the terminal & negative on the tread) rest it over the spout of a kettle and boil the kettle (you may have to hold the button down for a bit) it should light up the bulb shortly after the kettle boils. This test works on fan switches too
