The Self-Learn function worked as it said on the tin. You must take any batteries out of the device first and re-insert, as the function is time-limited. The Keypad registers it first as a Force Tamper.
V 6.5 and Portal is communicating 100% with the Panel. Honeywell have (very promptly) just sent me some new instructions that have not previously appeared in the Dimension Manual (their web-site and documentation always seem to be several months behind the pace) which look as if they should work. There is an Self-Learn function for the device's serial number, accessed it seems by pressing the * key and operating the tamper.