When you go for a ride yerself or with yer mates, you probably put all your attention where it should be - a couple of hundred yards down the road. It is very easy on these sorts of days to loose all your normal flow and planning, because all of a sudden your attention just isnt where it normally is - you're thinking ''I'm being watched', 'where is he now', 'Oooo, he's on that line, I wonder if I should be?' etc etc, and as a consequence your 'attention' spends almost as much time behind you rather than up the road.
It's hard, (and it
is hard at first ..), but completely ignore the bloke who's following you. Don't worry about his lines or his positioning, do what you do every time you go for a spin. keep your attention way way up the road and ignore him.
That's the best bit of advice I can give you.