Which craft? Likewise, I've been through Dronfield many times. I don't know if my suggestion will help, as I only have to deal with rain; riding through slush is never going to happen here. I use the old TCC suggestion - was it yours? - of the swim sandals and add waterproof socks. I use sealskinz and recently, I've bought through here. You have to pay for postage, but if you choose the heavily discounted stuff, then it works well. But as per the others, you can get water ingress at the top, in which case, I have two pairs - one for the journey to, and another for the journey fro - they don't dry out very quickly once wet inside. (I found the gaiter style thingies too much of a hassle.) And then just change my socks when i get there. Whether that would work out warm enough, I can't answer, though you could of course wear an extra layer of socks under them. But it won't keep your shoes dry, if that is the goal as well.
The other option for waterproof socks is to use wetstuit style swimming socks, as per the Directeur hinted at earlier. See here. The thermal ones should keep you quite warm.
On that topic, if the goal of the waterproof top is to keep you dry, rather than your clothes, then you could wear a wetsuit upper, like a DeSoto T1, under your clothing to keep you dry. Before you laugh, apparently some NFL players do this.