i remember reading it as a 46mm nut, but i cant confirm it as the haynes is in the garage.

It's just me being backward. I always think of say an 8mm nut being a nut that needs an 8mm spanner or socket, not one that goes onto a 8mm thread...

if i remember i'll measure it, if i can get at it, me garage is in a bit of eerrr mess at the mo!
Cheers
Ian

It is 46mm, just loosened mine off the other day

i know i need large leverage to remove this, would you recommend using a big torque wrench? or could that damage the internals of the torque wrench?

Don't use a torque wrench, they aren't meant for un-doing things.

I got my dad to sit on my bike with it in 6th gear, front and rear brakes held on while I used my 2 and half foot breaker bar on it.

Saying that I didn't really unstake it the best I could have.....

FYI

I did not manage to get the disc fitted without removing the hub.

Easiest is to use some for of impact driver/rattle gun...or failing that I use a 2 foot breaker with another tube onto it. No substitute for leverage.

okay no problem, il extend my current wrench, there are some hollow 2 meter long tubes il just slot that on the end of the wrench.... enough leverage :D

By wrench do you mean ratchet wrench or spanner? I'd be a bit wary about putting a huge bar on a ratchet for the same reason as a torque wrench. Personally, I'd buy, beg or borrow a breaker bar. Yes, more expense, but if you break a ratchet by extending it with six feet of scaffold tube, you'll be having to replace it anyway.