In reply to shurug: You could try nail varnish on the edges.. will stop the tear getting any worse. Clear stick on repair tape from a seamstress type shop will stop the down escaping.
Alternatively... my missus fixed a hole in her Helly with gaffer tape and its still going strong months later.
You can buy special self-adhesive nylon that is designed to repair... duvets. Simple really!
When you do it a bit of pressure with a cool iron will make the patch pretty well permanent. Last time I bought some I could only find black but I've seen different colours in the past. I've even repaired a rucksac with it and it's holding for the moment - a bit thin really though.
spinnaker repair tape. Yo ucan get the same stuff in b+q. Looks like ripstop nylon and yo can get the right colour. Maybe go round the edge with superglue so it doesn't peel off. Otherwise gaffa tapes makes you look like a proper climber
In reply to robin mueller: I did a good repair job by super glueing 500D rucsac patch material that Lowe Alpine used to sell with all their sacs across a pleothora of holes on mine a few months ago. The patch material clould be any sort of fabric really somethig cut off the side of any old sac or bag would do. Just thumb the down away from the site of the hole until the superglue has dried.