Thanks Chris!
I also had a few failures - it took 2 complete (apart from some small offcuts) of 160x100mm single-sided presensitised board, just to get one 22x30mm (ish) board completely successful! Most of that was trying to get the exposure right, though, so I expect future boards to go better. The other thing, I used sodium metasilicate (aka waterglass) as the developer, after reading an old web page (
http://www.electricstuff.co.uk/pcbs.html) which suggested that using plain sodium hydroxide was a pain in the arse; and, apart from when I'd overexposed the board, it did work a treat. So well, in fact, I made an entire bottle full of concentrate.... unfortunately, something's gone a bit wrong with it - see picture; it should be crystal clear, the white is a solid block of.... dunno what. I'm not sure if it's recoverable or not. Still... the raw chemicals are cheap, I can just make some more when I need to.
One of the next jobs I want to do is get the timer circuitry onto a PCB, so I can free up the breadboard for the next* project.
* Any suggestions from the floor?