Thanks guys. I nicked the strop already (god knows how, I was really careful!) so have had to sand it already. The wiki recommends 600 grit, but the finest I could get my hands on was Norton 220 wet/dry. Not sure if the 220 refers to the grit or what, but it felt about the same as a pumice stone, so I figured it shouldn't bee too bad. It took a bit of the dark brown finish off with it, so it's now chocolate brown with a few lighter blotches. Have I damaged my strop by using this?

I also rubbed a tiny amount of baby oil onto it to augment my natural skin oils (face and hands - good idea thanks FatboySlim!), so it's now velvety smooth. It's also got a fair bit more flex from a lot of hand-rubbing, so it's no longer like a leather board. Hopefully a couple of weeks and it'll be as supple as a belt or something similar.