Thanks for the response all. I got straight onto them again this morning. The return seats had indeed dropped out of the booking as they hadn’t processed the change in time.

Thankfully they were still there and I am now seeing the changed return flight in MMB. Now I just need a ticket.

The change fee thing is interesting. I’m a 100% leisure traveller and sometimes jump on long haul flights before I’ve finalised the itinerary for the holiday. 9 times out of 10 I won’t need a change but the ability to do so is clearly a benefit to me for relatively low charges.

If they up this then you obviously need to get it right first time, but then you risk losing the seats if internal flights don’t work out for you.