Thanks. Your theory sounds plausible, as it's a very basic bucket. I give the disassembled spigot a good scrub between batches, but there are bits in it that are impossible to reach properly. To be honest I'm thinking of upgrading to a stainless steel fermenter, and this was going to be my last...
I've brewed a few beers from kits, and they've all been great. I use minimal equipment - just a fermenting bucket with a tap - and I bottle condition. Anyway, my last batch (an IPA) got an infection, so I threw it out. Now this batch (a stout) is also infected - same thing, big gloopy pellicle...