They aren't easy to spot at all. The only place I see them on a forum is at rpoints where there is an arb thread but by the time they are posted, they're usually gone.
Best time is in the last ten mins or so before an event starts - the odds on BF will fluctuate a bit then and you can pick some scraps up. I think the thing with arbs is though you make a small amount of cash each time, it's little but it's often and more importently, there is no risk involved (unless the odds suddenly change mid bet!)
One problem I'm having with them though is that you need a nice BF account in order to lay off. I'm just starting out on a small BR and managed to offload £130 of my £160 BF account yesterday and now I have to go through the inconvienence of transfering it from a bookies account!
Good point about continental bookmakers though, try US ones too. UK ones tend to offer worse odds on UK things - shop about.
Oh and
www.oddschecker.com may help you a bit too - good for all sports at a lot of bookmakers.