So I picked up stitches all the way along the top edge and knit down, again in stocking stitch.
I had more yarn left than I thought and the resulting facing is almost three inches deep. Once I ran out of yarn I sewed down each live stitch with cotton thread that matched in colour.
Blocking did wonders for this. It really brings out the lace pattern beatifully.
The pattern for the lace came from a stitch book, it was called 'Branched Fern' there. Although the stitch is quite logical it isn't exactly intuitive - I had to keep checking the chart even after quite a few repeats.
I love the way this shawl looks, I'm just not as convinced about the facing. Maybe I should have tried some kind of band as an edging. Having hand sewn each stitch I won't re-do this. I will give a different design feature a go when I try something similar in future.