With my Exchange mailbox reaching breaking point I was despondently scrolling up and down my "Folder Size" results wondering where I should start trying to recoup some much needed space when "Thread Compressor" flashed in front of my eyes. It must be at least a couple of years since I've had it installed but I realised it could be a get out of jail free card in this case so went on a hunt for it....
Almost immediately I came across a post from the man himself, Ewan Dalton, asking for feedback. Buried at the bottom is a link to "a website" where you can get Thread Compressor. Installing isn't as easy as it once was - it has dependences on CDO 1.21 and some VB6 controls (back when I last installed it I probably had these on my machine). I ended up having to register Comdlg32.ocx, Tabctl32.ocx and Msflxgrd.ocx as well as installing CDO 1.21 and registering the threadc4.dll itself.