Remember that when you retune you should NOT select an 'add channels' option (if one is offered to you). You need to tell your TV to do a 'start from scratch' scan, as explained here:
http://www.nvtv.co.uk/freeview-info/
Unless things have changed, rescanning might change numbers. I've not experienced it for a long time now but I recall when I used to add channels, rather than start from scratch, channels used to be in unexpected places, and a subsequent rescan would change them back to where they were intended to be.