Thanks righthandrule - a very comprehensive reply, and I apologise for wearing my Mr Cynical hat. I certainly did not wish to imply that Jet2 was the worst airline in the world - it is far from that.
The only minor point with which I would take issue, is that the check-in staff at ALC did offer to re-allocate seats, but only if my daughter was prepared to pay to do so, despite having checked in online, whereas your post clearly states that this is not Jet2 policy. However, at the end of the day everyone was seated together, and I for one am now up to speed as to the seat allocation process.