Are these intervals from a stamped service book? Main dealer stamps? If so a main dealer may be able to get more information from their computer records as it's not unknown for a book to go unstamped in error even if the service was done.
One 17,000ml gap wouldn't normally worry me but the time period is a bit long at over 3.5yrs. Have a look online to see the MOT history and see if that gives any clue to its past repair history.
Overall though if the vehicle otherwise looks good and checks out I'd happily buy it with one proviso... check out their reputation for timing chain/pulley problems leading to engine failure and make sure yours isn't in the affected age range or if it is are there records of a recall repair etc.