as already said above couple of points if you get one on a reel make sure you unwind fully to stop over heating also some like the b&q one are only 10amp not 16 so will limit how much you can run at once.
I hope I've added the link right, but this is the one I've got - the Sunncamp triple mains plug.
Its got a circuit breaker and cover to keep condensation out.
The one piece of advice that everyone on here says is, don't buy a cheap one like the ones you can find cobbled together on Ebay - its to do with tents producing condensation and if that gets in the electrics its kaboom!!
This is definitely something you need to spend money on for your safety.