Thank you abergero, sorry for the tardy reply as my homelink inquiry was swamped by what turned out to be the dreaded RCM issue.

Your post will definitely help others in a similar situation to us. There's a lot on the forum about programming the homelink, but not much about if the unit is not working.

Another reply made a good point about powering and testing the unit with my garage doors before removing the headliner to reconnect it. If they don't work with homelink, there would be no point plugging it in.

I've resolved to just keep a door fob in the car. Not quite as tidy and nifty, but it works.