Personally I wouldn't try... let the next owner enjoy and buy a new one! In particular now is NOT a good time to move any tree and give the age, and therefore the size of the root ball your are more likely to kill it than not.

I know! Obviously it has sentimental value but the whole plot is going to have a house built on it, it would be dug out with a digger but not too sure

If it is going to die anyway give it a try, dig out the new planting area far largest than needed, put a lot of good rotted organic matter in the bottom of and then dig out the tree. Remember that the rootball is going to have approximately the same diameter as the canopy so make sure the hole you prepare is big enough. You will then have to water and give tlc to for at least 6 months, good luck but I suspect you will lose.

Thankyou! I shall keep you posted/informed on how it goes!!

But as has been said more than once recently, unless you specifically told the buyers that you were taking plants, if they are in the ground they are part of the purchase and now belong to the buyer and you have no right to move the plants, no matter how much sentimentality is attached to them, or what you imagine the new owners intend to do with the garden.

We've listed everything we're taking (most of the stuff) because there's going to be a house built on top of the pond, the fish in the pond have also been listed

well you have nothing to lose.
do you have a space for it already? if yes then dig it up with as much root ball as possible. lift it onto an old sheet so that you can wrap the soil around it then keep it well watered until planted and then well watered after that for the rest of the summer. Make sure the new hole is half as big again as the size its come from. dig in plenty of compost/manure etc to enrich the soil and to give space for new roots to grow out into.
good luck.

I moved one probably a little larger than yours it was in a neighbours front garden which was being dug up for a driveway and was only going to be skipped.
It took me a couple of hours to get it free digging under the root ball difficult because of the type of growth and did have to cut away some of the lower growth to be able to get it free, when free it was very heavy and took two of us to move it.
Didn't look to good after the move but has now fully recovered and look really good.
Give it a good watering after moving and don't let it dry out.

Everything we are taking including plants has been listed thankfully! Even down to the one remaining fish in the pond!