Got this to work finally.
F5 support thinks it's down to a known fault where the CMP doesn't get properly updated when you edit the iRule through the GUI (not sure if that means the iRule editor or the config. interaface but I use both).
Obviously, the CMP status as displayed in the console didn't match what the F5 was actually doing. By doing a "bigpipe save" in console and a reboot from the front panel, everything seems to right itself (after setting the tmm count back to zero of course).
There is apparently a hotfix for 9.4.6 which solves this problem.
Cheers All.