By my tables you would need to add 970ft to a 4000ft platform at -40 which is a huge error if not being corrected. -20 is about as cold as I have to deal with but it's still enough to need a 90ft correction to a 600ft MDA.
Sometimes basing the correction on a single point (as the tables do) can be misleading. It's the average temperature deviation in the column of air between the reference station and the aircraft that matters. Even over Alaska, where it can be ISA - 60 degC on the ground, the air is unlikely to be much less than ISA -30 degC (i.e. -25 degC at 5000 ft). That represents about a 10-15% correction for most enroute altitudes (as opposed to IAP levels, which are often close to that very cold surface).
Nevertheless, like Spitoon, I'm intrigued by the apparent absence of guidance in the UK and some other places as to whether ATC should correct assigned altitudes to preserve terrain clearance.