http://cdn.nwstatic.co.uk/gfsimages/gfs.20140205/12/102/h850t850eu.png

Is this close to a record? 

I would be interested to know what the lowest 850hpa temp is from a variety of sources, from a canadian source -6C is pretty extrodinary, and I wouldn't be suprised if we get to -7C somewhere (easily cold enough for snow by the way for lucky westerners) 

I think the lowest ever 850hpa temp to hit the UK must be around -20C in january 1987. In a northerly the lowest 850hpa temp is probably getting on for -18C in shetland or the western isles.

I assume you're referring to the -5 stretching across the Atlantic?

Not really, I have seen that happen in mega siberian blocks in the north; I am more refering to the -5C isotherm getting from a canadian or american source to the UK. 

A figure of -6C or -7C is very unusual for a westerly source, so I am wondering if it is a record. Note anything from the arctic (even if it is on a westerly or SWerly) doesn't count, it has to originate from the american continent. I see two major barriors against this ever happening:

1) The atlantic very quickly moderates any cold air, despite the source being exceptionally cold (sometimes as low as -35C) even at the 850hpa level

2) Cold air crossing the atlantic is ascociated with cyclonic activity, so it is very hard to get a direct feed without the air being wrapped around lows and being forced over even more atlantic before it arrives in the UK; contrast this to a siberian esterly which, though unusual, can be direct.

Definately close :)

but baffin island is within the arctic circle, so can't techincally be counted. 

The problem though Quantum is you stated canada or the U.S, and at least a 3rd of Canada is Arctic with more of it being Sub Arctic.

Specifically I'm talking about air that leaves canada or the US from somewhere south of the arctic circle, i.e a westerly airmass rather than something from the arctic. 

i understand you, however that link at the top shows the -20 and below coming out of arctic and sub arctic canada, not regular continental canada.

http://www.cabrillo.edu/~crsmith/subarctic.gif

this map shows canadian sub arctic.

Sure, thats fine. I'm taking about cold air coming from that airmass over canada, rather than from a nearer arctic source which are quite different. mP north westerlies can produce really cold air, but a feed from the canadian sub arctic almost never does.