ok, solved it..... looked at guest's solution afterwards, turned out to be the same as mine except i'd reversed the roles of lemmings 1 and 2.
i had noticed the cracks in the pillar before..... the reason it took so long was that, for some reason, i'm very bad at thinking of solutions that go outside the main playing area. insane steve's "The First of Many", for instance, took me ages....... and on this one it just never occurred to me to climb up that pillar. shameful, i know. thanks for the help, guys.
EDIT: ok, what's up with level 22? i got 100% really easily but the level requires less. backroute?