Mounted my EDIS module today (bit parky in the garage to say the least!) to the ally panel over the bellhousing, does it need to be earthed or does it do that via one of the leads?

nope. its got positive and negative connections in the cables, and the signal cables (crank sensor, and another one that i can't remember...) need to be earthed at one end. the edis has earth connections for these too

The only requirement is to mount it where it won't get too hot - in the Escorts and Orions it was mounted over a wing, so a little way away from the engine.

Simply put - bolt it to a metal surface (to help it keep cool - but no great issue), keep it away from the hot and oily bits, and somewhere where it won't get soaked every time it rains.

I stuck mine to aliuminum pannelling over the pass footwell with silicon.