I picked up a bottle of BBlonde from Superdrug for £1.60
Here are the results on a Plus 4 keyboard - left in the February sun for 4 hours (i.e. not much sun)
Before, after a good scrub with washing up liquid.
After 4 hours in outside. Bblonde was spread with a brush on to the keys and thats it.
A good result and extremely easy and cheap to do!