KNOG FROG rear LED light Translucent or white £2.99 + 3% TCB and free returns via Collect + @ Chain Reaction Cycles
As a cyclist, driver and walker nothing irritates me more about cyclists, or as I like to call them, bike riders, more than muppets who don't have lights when it's dark!
Well - for under 3 quid you can get a decent rear light that makes you legal and more importantly allows others to see you.
These are surprisingly effective for what they are. I just leave one on my bike permanently in case I am caught out but have also started to run them in the day and you generally get more space from cars.
Available even cheaper with free water bottle on this deal £1.75 -