Just remembered,when dd4 was in hospital and had a fabric plaster dressing they used an alcohol type wipe to get that off. The nurse peeled up a corner and then gradually worked round directly on the sticky bit rather than saturate the dressing.
Thanks snorris, that was really helpful. I bought some babyoil, and that seemed to work quite well.
I've left the final/initial dressing on: it's just a thin strip over the actual scar/stitches, and it seems very stuck with matted blood and hair (hairy birthmark ), and doesn't seem ready to come off yet.
The replacement dressings I got from the hospital are useless and won't stay on at all, so I think I'll put some fabric plaster on before he goes back to school to protect it properly.