nobody can reliable foretell the future... but the euro is on the slide thanks to turmoil in Greece, and sterling getting stronger at the moment; personally, I'd wait. (There'll only be the odd cent in it so you've not got much to lose.)
You can't tell when it's a good time
If we could everyone would be a lot better off than they are right now, wouldn't they?
Personally, I buy over the year, simply because I do it on a buy monthly basis from my wages rather than save and buy. Sometimes I'm lucky, sometimes I'm not