I got up on the roof to check that the furnace vent wasn't blocked, then went and totally failed to get a replacement hose. Luckily it's designed to run vertically or horizontally, so there's a blocked hose at the top. I took that one off, and used the plug in it to plug up a short length of the buggered hose, then used that hose to replace the buggered up one - apparently the manufacturer doesn't like selling spares to DIYers, so 2 rubber hoses cost around $75.
The van's started idling at around 1500rpm, so I prepared for the worst and opened the engine hatch. The cruise control adjuster had come undone,so it had been slowly tightening up the throttle cable. Sadly it looks like the threads on the adjuster are crap, so I've put another nut in there as a spacer to get to the unbuggered thread. It looks like it'll work, and it's easy to get off now I know what the problem is.
The furnace was still throwing errors when I stomped away in a huff, but it seems to be working for the moment. We'll see how long that lasts.
In the evening we had a marathon Dr Who watching session, and we're almost up to date now.