Making progress, one step forward and two back. I've noticed that the old seals were fitted with the flat side inward, toward the oil. I was under the impression that the lip/spring side should face the oil?
I'll try the headless bolt trick when re-fitting, thanks. For this evening I've ruined one of the seals trying to get it over the groove in the gearbox output shaft so now I've had to stop and order more and will be more careful when they arrive.