1986 series 3 4.2, today car idles fine but dies straight away when put in reverse or drive, will keep running if i keep revs up when changing into gear.
Any ideas where to start?

 

Somewhat common issue.

What's your idle speed in "P" ?

For starters I'd clean the throttle body and make sure the throttle blade is set to the proper .002" gap. Make sure the ducting between the air flow meter and the throttle body is secure and leak free.

Cheers
DD