Report the event to the Police immediately and make a record of the date, time and place you did this. Obtain a reference number. If you speak to a Police officer get his/her collar number. This is because claims against untraced motorists are paid by the Motor Insurers Bureau.
The rules of the scheme stipulate that a damage only claim must be reported within 5 days and an injury claim within 14 days or as soon as reasonably practical. Inability to report due to injury would satisfy the reasonably practicable test.
This also applies if you skidded on spilt diesel or some other contaminant, because the culprit in those circumstances is by definition untraced. A claim for injury must be brought within 3 years (as per the normal rules), but a damage only claim within 9 months.