frost protection for fruit trees in pots

If you want to cover them, I would go with fleece. Personally I wouldn't cover them but that's a personal choice. I may be tempted to fleece them if they were in bud/bloom & a spring frost was forecast.

Apple ,pear and most other non tropical type fruit trees need no protection from frost in the UK, the pots can even freeze solid and they will be fine

That sounds like great advice. I'm all for leaving well alone and not covering them at all - means I don't have to buy anything and get spend the money on another tree and pot to put it in!

Why can't all the books agree on anything to do with fruit trees? I suppose checking stuff on this forum is as close to definitive as possible! Thanks eveyrone

Just keep your eye on the temp when they are in bloom.

They will probably sweat if covered in plastic / bubble wrap although you could use it round the pot to reduce chances of freezing but to be honest I wouldn't bother.

Sweat.............that's the word I was looking for earlier but it just wouldn't come................

The problems occur if you're prone to severe winter freezes, especially if those freezes suddenly arrive after a relatively mild spell.
Pots freeze and thaw more quickly than the ground, which means the roots of the trees are subjected to more variation in temperature. On a sunny day the pot might warm up enough to encourage soft roots to grow, only for that lovely clear day to turn into a bitterly cold clear night that freezes and kills the soft roots that just started to grow.
Some rootstocks don't get tempted out of dormancy so easy, while some will withstand more severe cold, while others are vigorous enough that they can often repair freeze damage without the owner noticing much loss of vigour.
Dwarf rootstocks have low vigour, so the roots have less self-repair ability when damaged.
Smaller roots (or branches) are more likely to die from freeze damage than larger roots, and, of course, dwarfs have smaller roots and branches.

I read an interesting study from the colder parts of the USA and they found that in some cases it wasn't so much a scion's or rootstock's cold tolerance that allowed it to survive hard winters, but more a function of the variety's fast growth rate which allowed it to quickly re-grow what was killed by the frost.

When we had the week of -10'C to -14'C nights a few years ago, I had some young apple trees on MM111 and M25 rootstock in barrels (probably 100 to 150 litres capacity), and I believe they suffered some root damage because despite being in good compost and no lack of water, they didn't grow as well the following season as some which were in the ground all winter. Branches and roots weren't killed, but I think they were damaged and took time to repair themselves.

Conventional advice is to put feet under pots in winter so that they drain well - but that lets cold air under them, so they lose heat from the bottom as well as the sides, whereas there is some "heat" travelling up from Mother Earth which is lost due to the air gap.

If you think you need to insulate the pot I wouldn't insulate under it, but rather around the pot including a "skirt" down to the ground.

Other issue is that terracotta pots may well crack / break in cold weather

I would wrap something around the pots to reduce the swing in temperatures that will occur. You may think the pots are big but they can freeze all the way through after a couple of days of low temperatures. And that will damage roots, and the fine ones will be the easiest damaged.

The problem of pots is that maintaining a balance can be difficult, too hot, too cold, too wet, too dry can be simply different day to day.