What is the thing called what sends single to ecu 

You might need a little more description than 'the thing' 🤣

 

The ECU will connect to various sensors and controls on the engine.

 

It'll also connect on to the powertrain CAN-bus which is how it communicates with other modules in the car, including the immobiliser and steering wheel controller to start it.

Rac said canbus I think said it will cost about £50

Possibly the CAN-bus gateway?

 

They tend to be quite reliable but you'll need to check which version you have before buying a replacement. It'll also need coding in after replacing.