My friend gave me this Prusa Mendel (YAY) but the hot end is cluttered with molten plastic. I assume he accidentally used PLA of dubious quality. The printer was assembled in a workshop at the university of Bath in 2011, so the extruder is an early model. After reading lots of pages, I assume the hot end is a Camiel version.

What's the best way to go here? Replace the whole extruder? Get a new hot end, a recent version?

You mean clogged inside? Just heat it up, push some fresh filament in, then let it cool down fully. Next start heating the hotend again, while pulling firmly on the filament when it gets to over about 100C. You'll pull a slightly-molten plug of plastic out, with a load of detritus stuck to the end. You may need to repeat this a couple of times. I do this about every 100m of the cheap filament I use for printing, and it works a treat.

Sounds good, I'll try that. Thanks!

I've found that Nylon trimmer line works really well for this too. It doesn't melt at the low temp PLA melts at, yet molten PLA will stick to it like glue. You can 'fish' nearly all of the PLA and crap out of the nozzle with nylon at only 160C.