FlightXML3 AirlineFlightSchedules paging issue

I’m trying to get the flights for Southend for the next 7 days. If I call:

https://flightxml.flightaware.com/json/FlightXML3/AirlineFlightSchedules?start_date=1506338551&end_date=1506943351&destination=SEN&howMany=30

I get 15 records returned:
{“AirlineFlightSchedulesResult”:{“next_offset”:15, …

If I then call:

https://flightxml.flightaware.com/json/FlightXML3/AirlineFlightSchedules?start_date=1506338551&end_date=1506943351&destination=SEN&howMany=15&offset=15

It returns the same 15 results from the original request. If I request a smaller number than 15, (e.g. 5), then I get the requested number returned, but I can’t then get the next ‘page’.

Any ideas what I’m doing wrong?

The starter plan does not allow for offsets so the offset is ignored until you are a bronze plan or higher.