Freehub has started slipping, what can I do? 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:

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Cold also makes it worse, but this covers much worse temperatures that the UK: <

(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.)