This is with PicoScope 188.8.131.52.
Further investigation of the problem shows that the ext trigger is basically _always_ triggering (from my description previously, this is actually a different symptom, but I didn't investigate thoroughly last time). If ext is set to be the trigger, whatever the input, it triggers seemingly as fast as it can. If it is used as one of the logic triggers, it is always true.
This could conceivably be a software issue - I only noticed it since upgrading to the new version, but equally this could have existed prior to the upgrade (which was from a version from mid 2013).