HOWEVER...right now it has a real problem. The seat post is jammed solid. I have truly mashed up the alloy post trying to trist and saw it out.
I am about to send it off to Mercian cycles (the only shop who confidently said they could remove it). Let it be a warning to you all to remove and grease yours before it's too late. 4 years and 4 winters without moving mine was a bad policy.
Hopefully Mercian can get it out because I need my winter workhorse back before November kicks in.
An update:
When I asked for details from Mercian it turned out they were going to use a torch to melt the old seat post out.
This would have meant a respray, suddenly £25 for removing the seat post became an expensive option.
So, after reading about the Alloy-dissolving properties of Caustic Soda, I spent the last 7 days repeatedly pouring a highly concentrated mix of the stuff into my seat tube. This morning I was able to extract the remaining bits of foil-thin seat post (most of it, probably 15cm, had totally dissolved into black gunk which I rinsed out each morning.).
Caustic Soda attacks alloy but not steel (so this is not a good fix for Alloy frames).
Tips (for anyone who comes by this thread in desperation):
1 - Blu-Tack makes a perfect "bung" to seal the seat tube at the bottom bracket. Caustic Soda doesn't seem to have any effect on it.
2 - I used 150ml of water to 150g of Caustic Soda granules. This is VERY concentrated (Drain-cleaning strength is 500ml to 100g)
3 - When you add this sort of mix to the seat tube it all gets VERY hot and VERY bubbly, I advise you wear industrial rubber gloves, boots and goggles. Boiling hot Caustic Soda in the eyes is probably a recipe for permanent eye damage.
4 - Have a running hose-pipe handy to rinse off the spillage as it boils and bubbles from the seat tube. My paint seems to have survived but I made sure I was hosing the frame for the initial 10 minutes (when reaction is most extreme).
5 - Use an offcut of seat post to experiment with first and get the mix to a good strength. It's reassuring to see that it's working. After 3 days I wasn't convinced it was going to work but my offcut piece started to show real signs of thinning.
Good luck and take care.