Firstly full recognition has to go to Strobist site, that shared how to do this within the US.  On the same theme, follow these steps in the UK.

1. Go to Lee Filters website & using their Contact Us page (click top right “contact support”), ask for a free sample pack of their coloured Gels.
2. Within a week you should receive a sample pack of about 300 different gels.  Amazingly enough these are just about big enough to cover the front of a Canon 580EX, Nikon SB800 etc.
3. Dismantle the pack & take care to keep the piece of paper behind each gel you want to use, which described the gel in full
4. Cut the paper description in half, just to the left of the description.
5. Place the 2 halves either side of the gel, overlapping by say 3mm
6. Using clear sellotape, now fix the paper strip to the gel.  I overlapped the selotape by about 5mm on to the gel for a firm fixing.
7. Wrap sellotape around the rest of the paper, to provide a durable coating.
8. You should now have a gel that if you place in front of flashgun & bend paper sides, around the sides of flashgun, you can simple use a rubber band to hold the gel in place.
9. Happy Strobing

Other links that may be of interest

Softbox mount for your strobes / flashes – Total cost £1.48