In case anyone finds this during a search this is what I found out/did:
Outdoor equipment repairers wanted about 25/30 quid to do the repair so instead I got nylon thread (more stretchificated than cotton) and some two way stretch nylon from a haberdashery. I then sewed two patches on, one on each side of the fabric. I did this whilst wearing the tights, firstly sewing the edge of the patch whilst making sure the patch was stretched to a similar degree to the fabric. I then sewed the edge of the hole to the patch ensuring it is evenly stretched (look for the lines in the fabric and keep them straight). It's worked really well.