Andy B is right , 46mm, and spookily is the same size as the ball joint nut on an old mini, so most decent motor factors will have them (i.e. not Halfords).
Without trying to teach my grandmother to suck egss and all that, also worth cable tying the circlip when you reassemble, as these rear wheelnuts have been known to work loose !
You need 46mm for the wheel side and 41mm for the sprocket side.
Halford can get them with 2 or 3 days turn around. They order them from TENG TOOLS and they are about £16 each in satin finish (chrome is much more expensive, about £25).
Those come with a 3/4 inch drive though and you will need an adaptor (available from Halford too) from 1/2 inch to 3/4 (about £8) since I suppose your spanner will be 1/2 drive.
Also you will need a heavy duty Torque Wrench to operate that. I got a cheap one from ScrewFix.com (about a tenner). Not worth splashing a lot on such a wrench since this is pretty much the only time you are going to use it. Better to spend the money on an acurate 3/8inch drive one.
Also I have bought a handy little Jump Start system from screwfix. Really cool to bring on track days. You charge it up at home and it provides power to jump start the bike (sometimes a problem on Dukes lets face it), light, and even a small tyre inflator! (about £50).
Its such a long time ago that I fitted my DP hugger, but I can definatly say that I didn't remove the wheel, all I needed to do was remove the caliper so that I could thread the brake hose.
thats a good point...what make is the hugger? Do you really need to remove the wheel. I know when I first got my BIP, I swore blind I would have to remove the wheel to get the old hugger off...but alas no...just a bit of jiggery pokery and out it came?!?!
Mood: My Jota goes rumpety rump! & I have my wife exactly where she wants me!
I would suggest you remove the rear wheen just to make life easy and i would also suggest you screw and tap the arm to do the job properly, its very easy. Good Luck