Id go for boarding it out steve with the gaps for venting, dont even bother with insulation mate. see how that goes. At the end of the day, even if the condensation still exists it would take YEARS for the small drops to rot the boards.....

Old houses...like cottages and stuff that hardly have any ventilation in the roof space have condensation on the underside of the felt which drips onto the plasterboards and generally doesnt cause too much problems....so im sure your garage will be fine.