Yes. You definitely need to do that within the required timeframe. However - be very careful - if you plead guilty, then it's game over. I'd be persistent trying to get hold of them - faxes, phone calls, emails etc.
One thing I should have mentioned as it's often overlooked: how long ago did this happen. There is a six month time limit to lay charges in the Mags Court - if they were outside this time limit, then you're onto a good thing...
Malcolm - thanks for your helpful advice with this. I've managed to avoid the summons but have been issued with a warning fare notice instead. Would have preferred not to have got this but accept that under the circumstances it could have been worse.
Thanks again.
Maggi