Dwarf Apple Tree

The best time to re-pot would be while it is dormant - December-February.
It prevents roots being damaged while the plant is using them to supply water and nutrients to the leaves.

If potting to a larger pot, you can do it at any time, but don't knock more than half of the compost off the rootball while the tree is growing - and don't leave it out of its pot for more than a few minutes if it is in leaf or the roots will be damaged and the leaves will die and fall off.

If the flowers aren't turning into fruits, it seems most likely that the bees aren't visiting to bring compatible pollen from another (different variety) tree, or at least rummaging around in your trees flowers to scatter pollen from the anthers (pollen/ male part of the flower) to the stigma (female part of the flower).
If a tree has only a few flowers, the bees won't bother to visit. Bees like large masses of flowers so that they can harvest pollen and nectar in bulk, to make their trip worthwhile. As apple trees mature, they should produce more blossom each season, although in the early years they may not set fruit or may drop the fruit before it ripens. Young apples trees are fickle things.

Initially, you could try keeping it in a sunny but sheltered spot where the bees can easily find it.

If pollination still fails, you could try "tickling" the flowers with a feather (be gentle or you will damage the flowers!) to scatter pollen around.
You could also get a suitable pollinatin partner (note that some varieties are poor pollen producers and certain others are not compatible with each other).

Which variety do you have?

Thanks. So I could just lift the plant out and put into the centre of a bigger pot and put new compost round it so it would barely know it's moved. Would it be better to go straight into a vey big pot or just go a little bigger and then go bigger again as it grows?

It only has a few branches and only produces a few flowers so that's probably why the bees don't bother visiting. I would say it has produced more blossom each year but there still isn't much. I'm afraid I can't remember what type it is.

Hellbell

If the tree is a bit slow, you may want to go to a larger pot sooner, so that there is more room for it to grow roots. Restricted roots will reduce growth - fig trees are actually planted in slab-lined pits to limit root growth and control tree size and improve cropping.
When you re-pot, loosen some of the outermost compost to leave a few roots hanging loose, so that they can be spread out into the new compost and roots can quickly start growing.

Growth can be increased by giving a feed that is high in nitrogen and ensuring that the compost doesn't dry too quickly after watering.
Note that many plant feeds are more for the benefit of flowering and fruinting and therefore rather low in nitrogen, to discourage growth and promote flowers.

Thanks FB, you've been a great help. I will get it repotted and fed, give it a little tickle and hopefully will be eating my own apples next year!!