Steve, I’ve suffered from the same problem on the road bike for years, tried all the socks, boots, overshoes etc with no real improvement. Last year however I tried merino socks, sandwich bag over these, my normal riding shoes (ie summer ones with a bit of room in), but with battery powered heated insoles in. You can get them on ebay for about 20 quid, and the battery packs strap to your calves so wires don’t get in the way. Combine this with neoprene overshoes, and it is by far and away the best solution for me.