Hullo Mate
On a quick set if you shorten the running line you'll shorten the net so you'll need to replace the missing piece or replace the whole line (on a 100yd net that'll be 200yds plus twice the working hieght of the net, better to go a bit too long than too short)needles and twine you can get from Brian Brinded, Bridport Nets, Agouti, Netloft, google 'em they've all got websites.
Brian Brinded did make an excellent DVD Net Making and Repair, well worth the money a very good watch, I think I paid a tenner for mine
ATB