Looks like Sticky Back it is then Geoff!

I've ordered some yellow sticky back tape for future minor occurrences and Sam's Sails has it on the roll 1500mm wide for about $50/m (sold in 0.5m increments).
Thanks Essedon, I've just patched another sail with Gaffa and it held in 25knots on the weekend. Colour matched might be slightly more classy.
Thanks for the tips Relic. Stabilising the tear with masking tape makes good sense.
She'll be good as new! (well maybe not, since its a circa 2007 sail, but was going strong prior to this).