Spotted...

Quick undertaking question, what do you do when the rider infront of you is a) riding slowly and b) riding on the right side of the road, so there is no way to get round on the correct side without going into the oncoming lane.

I know its highly frowned upon but how are you supposed to get past such numpties without undertaking?

You would move into the oncoming lane when safe to do so (just like you would if you were in any other vehicle). If it's never safe to do so, then you've got the option to either suck it up or be a dick.

Edit: Or you could just politely ask 'em to move
Edit2: beaten to it by BR