thanks for the reply, I got a feeling it's something to do with apt-get.
when I apt-get install kernel-source it says it has installed it but all it does is place kernel-source.tar.gz into usr/src/
whereas installing kernel-source in mandrake untars the file & places modversions.h everywhere it's supposed to be (usr/include/linux/ I got it wrong in my initial post
Manually untarring the kernel-source & copying modversions.h doesn't help, the drivers just complain of loads of missing files... is it apt's fault? or mine (much more likely!)