This site has some really good info on making rockets. My release system is based on the videos here.
For information, the B&Q own brand (£1 a pop!) hose connectors are essentially identical to the Gardena system ones. Don’t bother with Hozelock though, as I ruined two connectors trying to take them apart – there doesn’t seem to be any way to get them apart without breaking the retaining clips 😦 (luckily they are on special in Tesco at the moment so didn’t cost much!)