Morning, it's not as cold as that where I am, but have had minus one and 410 seems to love it once you've got some warmth in the engine and gearbox oil.
I don't know if it's the same but in Austin's and similar that I've owned with eyeball vents the sealing inside the vent was accomplished by bits of felt that over the years became loose/moved slightly and prevented it closing/sealing properly.
The Jensen fellows have a solution which may help
https://www.martinrobey.com/jensen/i...on-cover/15104
I've managed to get them working in most cases (as suggested below) with judicious use silicone spray to get the parts moving smoothly so they could force their way closed.
Other solutions have included wearing a single glove and blanking the vent off until weather warms up. I appreciate that these solutions may be acceptable in an Austin 1800 rally car and less so in a Bristol!