I'm probably basically with MM on all the points, but here goes.
1. First of all, I did an inspection as it was quite sunny today & the bees were pretty active - at what point does it get too cold to go in to the hive? I read somewhere it's when it's 12 degrees or less outside - is this correct?
Good books don't generally tell lies, as they are carefully checked and double checked for ***** mistakes - err, I mean accuracy of data. There are a few out there that are short on real facts in places. Make sure your books are from a responsible and well respected origin. It is mostly the new books written by people trying to jump on the 'bees are at risk' band wagon just to make a quick buck.
2. I have a hard floor on my national - should i buy an open mesh one for better ventilation over winter, or will they be ok without?
As I say lower down, OMF and no top ventilation at all. But solid floors have been in use for the last 150 years so can't be that bad. Balancing ventilation without through-draughts is always an art-form and I am no artist. I suffered either damp frames in spring or too much chimney effect. Neither of those can be seen as good for the bees' welfare. Also I am on jumbo frames so deep enough for the bees to install themselves further away from the OMF. These are similar to a brood with super over. Oh, and no problem of driven rain entering and remaining in the hive exacerbating the damp conditions with an OMF. I don't have to make so sure the floors are sloping slightly down towards the entrance.
3. Mousegaurds - what sort to buy & when to fit? I keep hearing conflicting opinions, that wooden entrance blocks are better than metal gaurds as the bees have less gaps to gaurd???
Good ones fitted as late as possible but obviously before the mice are already installed. Mice will not gain entry until the bees start to cluster, any more than other aliens (apart from wax moth?) to the hive environment. All may mean losing some pollen, foraged at great expense to the bees at this time of the year, so again it is important not to put them on too early. There again, you may be intending to help them with pollen substitute when expanding in spring. This is where the pollen will be used - for spring expansion - and a dearth will slow down the colony or may lead to starvation if honey is exhausted during the expansion as well. Get a good quality one for a start - after all you will only need the one this year, so not a great expense considering that the rodent on board will likely lead to the death of the colony.
4. When to stop feeding?
Feed fast and stop when the space is full. No half measures or there may be uncapped honey or too much space at crownboard level. If too late, you are too late and should have done it earlier. Spare frames of capped honey at this time of the year can come in very handy. I overwinter on a full 14 x 12 brood. If it is not filled I remove outer-most empty frames and squeeze the rest to one end with a divider or to the front with divider behind. Divider is air tight or thereabouts at the crownboard, so no warm is lost that way. I am on OMFs so no top ventilation at all.
5. Woodpecker protection - is wrapping chicken wire around the best option? Or tying refuse sacks around?
As good as any other and better than some. It needs proper application to be effective, of course. Just 'wrapping it around' may not be sufficient. Woodpecker damage is a nuisance and can be fatal to the colony but I have only ever had one incident (last winter) where the hive was attacked but not breached. I fitted a heavy, close-fitting plastic bag over the brood but under the roof and that solved the problem, if there was going to be further damage, of which I could not be sure.
Hope this lot helps in some small way.
Warm, dry, healthy bees (and enough of them) with sufficient stores for winter (and a late spring, if not fed at that time)) with protection from rodents, birds etc. are the real keys/requirements to overwintering a colony and it getting a good start in the spring.
Regards, RAB