I've just gone through the london winter and both my bikes now have a slipping freehub. Admittedly my mountain bikes' freehub is quite old and probably worn out, but I'm wondering if they are seized up inside and I can apply some lubrication to stop it from slipping? Anyone got any good advice?
It can happen that the pawls get sticky and cleaning helps. Here's a couple of guides:
Cold also makes it worse, but this covers much worse temperatures that the UK: http://www.icebike.org/Equipment/freehub.htm
(And while it's true that "flushing with solvent has not been a reliable substitute for full disassembly", if the freewheel isn't engaging at all, just drenching it with WD-40 can work as a short term fix until you get round to a proper job.)