I was thinking a leak too, maybe when I moved the caliper out the way I split the hose. I'll get someone to jump on the brake pedal while I stick some paper around the hose to check for leaks.
Right sorted, and by sorted I mean there is still a problem. I took the battery cable off and reconnected it. The lights haven't come back on. My guess is there was a stuck relay somewhere, causing the crackle sound.