I thought it would be sensible to check with Merlin's previous owner whether he'd ever been clipped or not and the answer was not as far as she knew. So this morning I borrowed Mick's beard trimmer, took it out to the field and switched it on while Merlin was standing next to me.
There was a lot of ear waggling and he took a couple of steps back, but after 20 minutes of switching it on and off randomly while I was shovelling muck he was ignoring it and he didn't seem bothered when I put the hand with the trimmer in against his neck so he could feel the vibrations, though he did step away after 15 seconds.
All good anyway - if he has been clipped in the past it certainly wasn't a bad experience for him. I'll keep the trimmer in my pocket and hopefully come next Thursday he'll be completely unbothered when they shave his legs.