I've noticed over the last few days that the freewheel on my singlespeed has started sounding rather gritty while coasting. I was wondering if anyone could give me some tips on servicing/lubricating it or whether it's a job for Cycle World.

might just be grit/dirt on the sprung loaded bits inside hub and on the freehub, u dont state whether its sealed or taper bearing'd wheel, i've only done campag (sealed type) but i suspect cone bearing'd hub is the same,
strip the hub as if u're takin the bearings out and the freehub should lift out the centre of the wheel in one piece, should be able to clean the teeth inside the hub and inspect the sprung lobes on the freehub,i've never stripped a freehub apart but u should be able to clean and spray lube to quiten it,hope this helps
Mark.

It's a shimano screw on single speed, having had a look at chainreactioncycles they go for about £15 so I might replace it once the weather improves a bit. I can feel it dragging a bit as I freewheel, which is my excuse why I was passed on my way down Portsdown hill yesterday.

Ah! Shimano stuff - not designed to last in this disposable age....