Soooo, I have ordered an alarm called a Packalarm from the US. This is a self contained reel holding 330 metres of line which connects to a trigger on the unit. Looks pretty good, let's hope it is!
You can see a video of how it works here
The Packalarm arrived in the post today and I am very pleased with it. It is compact and light, activates very easily, the line seems perfect for the job and the unit is simple but seemingly robust. Apparently getting wet won't affect the unit which is great so I needn't worry on rainy days.
I will set the trip wire up again in a day or two and see if it is as good as it appears to be.