l know the easy bits just need to locate the coil or some ECU wire

l have never been to an auto parts & accessory shop in the UK. But in America, there r places that sell all sorts of guages & meters to ''add on'' to ur car. Aftermarket guages.
l had my choice of 5 different temperature guages when l wanted one.
Could u talk to a salesman there, in the UK, & buy one with complete directions?
l recommend staying away from the ECU. That is a very expensive thing to mess with. Try adding ur tachometer to a spark plug wire, or the coil as u stated.
More complicated ones (and cruise controls) have spinning magnets on the flywheel that register the revolutions.

l am currently putting the temperature sensor & dashboard guage from a 306, onto a 205. They have the same water neck with thermostat.