the boiler may be recieving it's pernament live from the plug but its switched live from another source (external programmer, motorised valve etc). In this case remove the plug and you remove the earth but still have a live 240v to boiler. NOT GOOD.
I know that its not in any regs, but most boilers will still have an earth connection via the gas supply pipe which will be copper back to the earthed bond by the meter. I get better trainees to note that.
Tony