Hodster - we are in exactly the same position. There we 8 of us booked, good flight times, booked early to get reasonable price then this happens! Doubly annoying as another 3 of our party are flying from Manchester at what was the same flight times to share transfer - now we are going to have to pay tow transfers - Jet2 don't factor that in, nor the higher cost of booking an alternative at this stage. It's not only missing out on our first night, it's the time filling when we depart - moved from 5pm to 23.55!! Don't think I can afford an extended time in Dalaman Airport! Ho hum. Am now going to book with ThomsonFly - leave at 07.35 arr 13.45, so raspberries to Jet2. They just lost 8 fare paying passengers. I wouldn't mind as much (that's prob not true actually!), but it's a big bash for my birthday and I have splurged on the White Lodge for the week as a treat for me and my family, so want to maximise the time we have there!
Anyway, hope you get sorted and happy hols.