Finally found the paperwork from my dealer for the £250 plan.
"Once you've made a payment of £250, your first 3 services are taken care of.'
12 months/10,000 Oil change service
24 months/20, 000 Interval service (including oil change)
36 months/30,000 Inspection service(including oil change) and brake fluid change.
So ignore what the service interval on the car says and take it in every 12 months. Make sure the 3rd service is just before the 3 years are up or you'll miss out.