Richard,
Oh dear - sounds like you fell into the classic trap - we've all done it at some point.
To be honest, I am not quite clear exactly what you need to cut but it sounds like a job for a handsaw. A properly sized saw need not be expensive and will cut easily and accurately.
I know it's not what you want to do but I at a country fair a few years ago, I witnessed a couple of experts saw through an eighteen inch log with a two man saw in SEVEN seconds! Mind you, the teeth on that saw were as big as my fingers.