Understood. When I said 'swapped in a 165' I meant 'stripped the breadboard and started again with a 165'. I rewired for the differing pinouts each time I've changed chip.
I'm frustrated about the 166 situation. It should work. I'll go back at some point and try again. We've got 166s ready to go so it seems a shame to have to go buy 165s (even though they're not expensive).
I've not used decoupling caps as yet so maybe that's an issue with the 166. I'll have a root through my parts bins.
Thanks for the heads up on that.
[m]