Crossroads - thanks for the suggestions on producing schematics.
MurMan - thanks, all your points taken. I stole the idea from Gratulacje! Twoja domena została zarejestrowana w OVH!
and that is why it is set at 3.3V and also why I am using the TLS261 as I am. When I switch to 5V I get a swing from 19mV to 3.92V which is more obvious than the 2.7V I was getting before. The best bit is when I switch to 5V the circuit works as expected and the interrupt only fires when I release the switch to the LED. I'm sure I've got some LM339 or the like lying around which I can use for the comparator.