Got one of these on my cx bike:

http://www.chainreactioncycles.com/shimano-single-speed-freewheel/rp-prod7805

Did a big ride yesterday, full of rain,mud and grit – by the end of it the free wheel was just grinding – cranks won’t spin backwards freely and it sounds bloody awful. Are these serviceable, or is it just buy a new one and replace?

I’m not sure they are serviceable. If you want a serviceable one, you’d be looking at White Industries for about thrice the price; it’s brilliant, by all accounts. The Halo Clickster also gets good reviews, although i’ve no first hand experience…

Charlie has a good selection.

The piece with the two small hole used to unscrew with a pin spanner, then you could get it apart.

Binned mine after 7 months on my day1 after it started bleeding rust (something about riding through floods).
Got a used white industries on the bay which has been great

Cheers guys. I’ll look to replace. Tempted by the white industries but it is expensive!