The "sixth sick sheik's sixth sheep's sick" is said to be the toughest tongue twister in the English language.
I wouldn't call it tough, just tedious. I can say it just fine without a single slip of the tongue, but all those consonant sounds running together take a long time to pronounce as far as pronunciation goes, so it's really only difficult physically and not mentally like most tongue twisters are.
Can anyone tell what I've been watching?
I didn't know there were other Doc Martin fans in the community. Welcome to Portwenn.