OK, so just in case anyone has the same issues in the future and searches the Forum - this is what I found was the best (only?) way.
1. Soak the 3M foam/tape in WD40 and leave for a while
2. Take a brand new blade from one of those segmented work knives, put plasters on your fingertips to protect them
3. Slide the blade in a gentle sawing motion lengthways down the piece to remove the tape - on one side it should pretty much all come away, on the other side you will tend to dig into the plastic so can only remove half of it.
4. The WD40 has no affect on the adhesive, but it makes the blade slide easier and keeps it sharper.
5. Then using the same blade, go crossways removing the remanents a piece at a time.
This removes about 98% of it and as long as you wash and dry the pieces to remove the WD40 then it looks like the new 3M tape will stick perfectly.
Thanks all for the suggestions