Have you tried taking them back to the shop?
Yes, I have a few times. They're excellent (Selkirk sports in Golden, BC) and I will get them to have a look at them again this season. These days my feet seem to change a lot more over the season, the right one in particular. The first season was fine without the x strip (my mistake, it's foam not neoprene), then the x strip was put in last season. The movement only became an issue at the end of last season and I think it's starting to compound the packing out of the liner which has seen over 100 ski days now. I had the same issue with previous boots bought in the UK.
I was considering getting a new cold temperature liner for this season anyway so I might give that a go and get them to do remould of the shell. But I didn't want to do that for at least a few weekends, so my feet get used to being in ski boots again. Thanks for the detailed help.
In the meantime your tips are interesting. I wasn't aware of the heel retainers or the ankle booties. I will probably give the booties a try and let you know how they worked out. That might prolong the life of the existing liners which are fine otherwise.