I have moved a few and they have surprising small root balls. Unless it is very old you should find the roots are fairly shallow (30-40cm) and certainly only in the top soil, they may spread a bit horizontally but these are easily cut with the spade as these won't be very woody.
On a side note they are shallow rooted because they evolved to grow in rocky situations hence their love for demolition sites. Some potted Buddlejas, especially davidiixweyeriana hybrids, have roots with a fantastic scent - a combination of rose and earthy pepper.

Thank you - that's a great help. I might get it out before my open day!

I have found that, although they have shallow roots, they like to hang on to the soil, I took 1 out a couple of years ago, that had only been in place about 5 years, it took me a good half hour to get it out, [mind you I am getting older, lol] Derek.

Me too - but OH is very good at removing things, so I've mentioned it to him. We make a good team! I'd like to get it out before my open days which are coming up fast, as it's not a pretty sight. :-(