Thanks Joe,
I've just double-checked with the code that disables call diverts.
I entered ##004# into the dialer, pressed call, and was greeted with the correct "MMI Code Started' flash, followed by the alert box, "Call forwarding // Erasure was successful".
Using a task shortcut to run the task (which does exactly the same thing) at will rather than on a schedule opened the dialer but never gives those messages.
There seems to be a difference in what Tasker does when dialing and what happens when a user actually presses the keypad buttons.
I'm on an HTC Desire HD with an OTA upgrade to Android 2.3.5 and HTC Sense 3.0. It has worked for months since my last software upgrade, though.
Many thanks.
J.