IrregularPulse - thanks for the tips. I actually just knew the bucket needed a really good clean as it had sat open for a week or so and had been damp.
Turns out I completely buggered the whole thing up. I ended up with only 3 gallons of wort and was just way too tired after 6 hours of hectic brewing to go for a third sparge, so I topped it up to 5 gallons with water, took a hydro reading and got 1.028!
This is NOT what I wanted to see. So, I disolved a jar of orange-blossom honey in some boiled water, and added it to the wort before pitching the yeast. I then got an OG of 1.032 which is fine for what I'm brewing (a light summery pale ale).
This brings me to the question:
Why, oh why, did I knowingly only mash/sparge with 5 gallons of wort if I knew I was going to lose a lot in the boil? (my kettle has a really large surface area). Oh well, this is the first time I've used this new equipment (5-gallon boil kettle) and the first time I've tried all-grain. I know what to change next time to fix it - and the first thing is making a proper cool-box mash ton!