Is it safe to cover babies hands with built in fold over mittens (bonds zippy) at night? She is 7 months old and we live in a cold climate so her hands are really cold at night. Sleeps in a bonds zippy and sleep sack (correct tog for the room) but I have been leaving her hands out which I think are getting uncomfortably cold

Using mittens that are a part of sleeping bag, are not known to cause risk to baby, There may be some concern if she is sucking on mittens. It’s not uncommon for baby to have cold hands, especially during winter months.
https://rednose.org.au/article/how-much-bedding-does-baby-need

If your baby is settling ok, sleeping ok (for age) & not waking at unusual times during sleep (especially in colder months) she is probably warm enough.