put a heater in the back of the van mate and you ll have no problems.
i see this weather as a challenge instead of "i hate working in cold weather".its all about attitude and being organised with decent waterproof warm clothing.
since going wfp i find winter a breeze compared to trad(i was for 16 years!).
i use a cold system and it still allows me to work up to -2.any lower then i dont bother.its very rare its below -2 in the parts of the country where i work(manchester) in the day.
regards
dazmond