I used an ex-RAF growbag for my open cockpit backside freezing sessions. I bought mine from Mart Aviation. They used to have an advert in the back of Pilot magazine. I paid £34. I would steer clear of some flight suits sold by large Pilot supply shops (if you get my drift). Often these suits cost nearly £100. They advertise the fact that they are made of nomex, have pen pockets bla bla bla. You get all of this from an ex forces suit.
For wind beating and ultimate poser factor you can't do better than a WW2 Irvin made by Aviation leather craft. This is the only company that has the rights to carry on making an Irvin using the real pattern and Irvin label etc. I could never afford one (about £350 when I was looking.
The RAF leather gauntlets are another good buy. You will also need a pair of goggles.I bought the Halcyon WW2 replicas. They have the split screen, enamelled frames, adjustable bridge and brown webbing strap. They also look the part. These cost me about £40. You can also buy WW2 style leather helmets that will accomodate a david clark style headset. You will need a helmet as a lot of heat is lost through the old block plus wind in the hair can be suprisingly painful after a while
This will not help the bird pulling stakes but wearing long johns keeps you warmin the winter.
Bill