in the interests of anybody searching in the future - I've fixed this, using the following two sources:
http://forums.ross-tech.com/showthread.php?2059-SEAT-Leon-Mk2-Setting-the-Service-Reminder-Interval-back-to-Flexible-Schedule/
http://www.seatcupra.net/forums/showpost.php?p=4523024
The basics of it is that resetting using the trip button automatically puts the car onto a fixed service interval, by changing the 'oil quality' channel to the value 'poor oil quality'.
By changing this back to 'good oil quality', the car has automatically changed back to 18500 miles or 730 days (as it's been 3 days since I reset it, and the max is 730 days).
I then also altered the value for 'time since last inspection' to 14 days, so the MIB2 now shows 18500 miles or 716 days to oil service.