I'd wait till it has shed it's leaves over autumn, just make sure you take a reasonable sized root ball and wherever you replant it make sure it's not a cold, windy spot as it will suffer and give it some TLC over the following year. I repotted one about the same size back into the same pot - pulled it out, trimmed the roots by about 25%, trimmed about one third off of the top growth and repotted it with fresh compost and its doing perfectly well this year so far.
I helped my dad move one last week, it was a similar size, we dug about 3 foot across (1.5 foot each side of the main stem) and lifted it into the new position, first time i have moved one, but all seemed to go well and my dad has moved other like this in the past.
Branches can be fragile so be careful when moving it. Any lost branches will take a long time to grow back - if they ever do.
The people re-modelling our back garden said they would move mine, I was going to do it myself. They smashed all the centre out of it and I can't see it ever recovering.
Many thanks for all your advice.
Will proceed with caution when the leaves are fallen taking as large a root as possible to a well prepared and sheltered site.