Aarrrgghhh... I get the "Mks" and "Nºs" mixed up... I presume you mean a WWI vintage Lee-Enfield? (See
http://world.guns.ru/rifle/rfl04-e.htm; I'm referring to the Mk.III* or SMLE Nº1 Mk.III) If so, I went through the same thing with a barrel I had put on a Martini-Enfield as a replacement for the original one, which was shot out. (The threads are the same.)
Getting some penetrating oil under the parts for a few hours may help. The front sight band and base should come off by tapping the block forward, off the muzzle. There is a small "key" (through which the front sight pin passes) which sits longitudinally under the sight, fitting into a groove in the barrel and the sight base. IIRC, it has a tiny ledge at one end which stops the sight block from going back on too far when you are reassembling it; this also indicates which end is which.
The rear sight should likewise come off by tapping it forward, once you've got the cross-pin out as well as the small screw that goes into the barrel, visible by lifting the rear sight up.
A little heat might be helpful, too. if they are particularly recalcitrant. I didn't need to use any on mine, though.

Stuart