Viola! The Dremel makes short work of yet another case [Dremel says - "I own your ass".]. After smoothing off the rough edges with a grinding stone, I fitted the 120mm fan, complete with finger guard on the inside for digit protection. A perfect fit with a few mm's to spare around the fan - a huge improvement over an 80mm fan sucking air through that perforated metal grate.
The 80mm hole was offset for some reason, I fixed that by cutting the 120mm hole dead straight for a better finish.
FULL STORY @ PCPOWERZONE (http://www.pcpowerzone.co.uk/athlon_ultra_rig.html)