Honestly Fazersharp, mate. Learn to spend a bit of money where your favourite things are concerned (e.g. riding your bike). Get yourself one of those Pertex wind shirts, like the cyclists use. Thin as a fag paper, weighs nothing, packs down tiny in a little stuff sack. Put it on under or over a pullover, makes one hell of a difference without bulking you out. If you need to be warmer still, try a down or primaloft jacket under your bike jacket - some have a Pertex outer layer, so they are very windproof, and come in different weights, so you still don't need to bulk up too much. Wind chill is half the battle.
Or stop riding in freezing weather - which I can highly recommend
- saves on petrol and general running costs too. In fact, stay in bed all day, then you won't even need to pay for central heating