Why would the bike idle better with one cylinder rather than two?
I went for a ride and the bike felt really sluggish, but was idling fine compared to what it usually does.
Got back to the house and realised the left plug lead was off, so reconnected and now the bike won't idle as well, and is doing the hovering idle bit.
Thoughts?

The cylinder that was unplugged may be running lean.  Check the plugs.  Also, make sure that the carbs are clean.  The mixture screws also might not be set the same.