I think GO you are in Scotland, no?
I am Finnish and my family are farmers and I remember when it got cold (below -15C) we would wear old ski salopettes and jackets, and wool underneath in however many layers was necessary. Big insulated boots at least size up to leave room for toes to wiggle. These days ski outer layers don't tend to be insulated any more because people have cottoned on to the layering system, and because technical fabrics have developed and improved so much.
I would get a cheap waterproof set, www.fieldandtrek.com sells waterproof jackets for as low as £18, and waterproof overtrousers for £17. Worth having a look on their site, they are probably the cheapest for outdoor gear that I have seen. It's what is underneath that counts, and fabrics make such a difference so I would invest in those rather than waterproof shell, because if you have to wash it a lot you will not get more than a few years use out of it before the Durable Water Repellant coating gets wrecked - even reproofing doesn't really restore them to their former glory.
Wool first, and I'm sorry but I really don't get who goes around telling people here in the UK to wear silk, it doesn't insulate in cold weather. If you can find in the sale Icebreaker sets, the 400 weight midlayers are so warm. The dogs nuts is Woolpower from Sweden, they've added terry loops on the inside to increase loft. It's worn by mountain rescue personnel in Sweden and the likes. Or Aclima from Norway, www.nordicoutdoor.co.uk has a very good range.
On top of that, it's fleece, and tops made of Polartec Thermal Pro is the warmest out there so I'd look for that. You can get leggings made out of Polartec PowerStretch, like these which are amazing. Pretty much these same base and midlayers kept me warm in -42C last winter.
For boots - big wellies with lots of room for socks if you are working outside. You need fleece socks with terry loops on the inside, not cotton. My mum nicked my old Sorel boots though sometime in 1990 and wore them working outside in the winter until she retired last year. I find if my feet are cold it's really hard to stay warm overall.