I can't remember the exact figure, but you can turn back on your initial leg to get a fix before starting the diversion. I think it's something like 5 minutes of flying time.
So if you get the diversion mid leg and you're not over a good ground reference (ie moor) you can "hang a 180" and go back to that railway tunnel you flew over 3 minutes ago and initiate the diversion from there.
Best ask your instructor what the rule is, or wait for one to come along here.
There shouldn't be any surprises in JAR world, and most examiners will give you the line "I'm only there as an interested observer, and I don't want to fail you".
Final bit of advice, keep going right up until the end, don't slacken off on that final circuit and bugger up your flare, or taxi dangerously. It's too easy to think "I'm almost home" and let the guard slip. This is a common occurence in the "car world", where over a third of all accidents occur within 1 mile of getting home as drivers suffer from "switching off syndrome".
Best of luck
Camel Toe
"Over Macho Grande?"