Hi everyone. I am in dire need of some help and advice. My parks work fine, main beams work fine. As soon as I switch on to high beams, my parks and main beams stop working until I put high beams off. What could be the issue?

Hello and welcome!
I reckon it's the multi function switch which controls the lights has got a short going on from being old and worn. If not that check all the earth's are nice and shiny.

And just while your playing with lights, Mark gave me the details of the guy who makes replacement membranes ( $75.00) for behind the instrument cluster.
 
Just incase you/we  need it down the track, or for future refernce.
They have a website:
https://www.aeowparts.com/
 
Mostly mini, but has other stuff for escort and Capri, inc lucas stuff and eng bay decals.