Thanks mate! I didn't know that "Bob's your Uncle" was an Aussie term - it's amazing what I am learning on this site!
An update - I was totally wrong on the pesticide theory, looks like a newbee mistake (some would claim I have a kangraoo loose in the top paddock!). Today the hive swarmed. So the lack of activity and the hive being full of bees should have tipped me off when I recently opened it. I didn't se swarm cells at the time, but I didn't go over every frame.
The good news is, I caught my first ever swarm, it was a really fun experience. Put a queen excluder above the bottom board, and after the first few hours, looks like they are staying. They are only a relatively small swarm I think.
All good news so far, but I'm nervous about the main hive that issued this swarm - will there be more swarms leaving? Any tips on what to do next?
Cheers,
John.