Thanks Lakeland Lorry, that's a helpful link but as a summer walker I do all of that stuff already. Yet I know my summer kit would be woefully inadequate on a bad day between Nov and March.
I'm trying to work out how to upgrade effectively without i) overbuying and ii) overcarrying (unless you know of any sherpas in the Lakes )
As someone who regularly goes out on the Cairngorm Plateau in all seasons, here's my two pence worth.
Don't discount your summer clothes altogether, you can just add to them. Long Johns can be worn under trousers and over-trousers worm on top. For your top half, layers are the answer. Two light fleece sweaters are better than one thick top. Putting a thin pertex windshirt over your fleece increases the warmth considerably. You want to aim to be cold when you set out in the morning, if you're warm to start with you'll be too hot and sweating once you start slogging up hill. Having extra layers that can be put on over your other layers is infinitely better than having to take off your top layer to put on another top.
Hands need looking after, I carry two pairs of thin fleece gloves, a warm pair of ski gloves and a pair of mitts. On my head I have a thin fleece beany hat which more or less stays on, when it gets cold/windy/wet I put a fleece lined mountain cap on over the beany. When it gets windy in the Cairngorms and the snow starts to drift, it can be like being sandblasted by ice crystals. Making it impossible to see where you're going, even when it's blue sky's above you. The answer is ski goggles, I can't tell how many times I seen people staggering around lost just because they didn't protect their eyes. A buff is also good as it seals the neck and can be pulled up to cover the face.
Comfortable soft summer boots unfortunately won't be so good in winter. You do need something that will stand up to kicking steeps and stiff enough to take crampons. If you walk for any length of time in snow you'll end up with boots full of snow unless you have gaiters on. Love them or hate them they're the only way of keeping your feet dry.
On gentle sloping ground - where you won't go sliding downhill if you fall over - walking poles really help. Once it get steeper you need to put away the poles and have an ice axe in your hand. If you don't know how to use one to stop a fall you really should consider doing a winter skills course. An ice axe can stop you if you fall, but wearing crampons will stop you falling in the first place. In lower hills microspikes may be all you need but in the higher mountains crampons are better.
In winter navigation is harder, but the equipment is just the same as in summer. You just need to be far more aware of where you are - all the time. Daylight is shorter, it's normal to start and finish in the dark. Having a good headtorch is essential but you don't need to spend hundreds of pounds for one that's so bright it can be seen from space. I carry a spare just in case. At the bottom of my sack I carry another top, a primaloft one with a hood, a bothy bag and a first aid kit.
Of all the above the winter skills course would be the most important.