Blanking off a copper pipe

[onesoma]

I need to blank off two pipes exiting my hot water cylinder. It's a direct system, the pipes go to an old back boiler in an open fire I'm taking out.

A few questions:
1. What are the chances of me getting those nuts off with the threads intact? The cylinder may be nearly 50 years old.
2. Do I even need to get them off or would it be better to cut the pipe further away from the cylinder and fit a Compression Fitting Stop End Cap like this one: http://www.youtube.com/watch?v=0-6cbpzYO-8
3. I've never done anything to do with plumbing before  , should I give it a go or leave this to a professional? Fitting a compression fitting seems quite straightforward.

Cheers for any advice  (hoping the image attachment works...)

Edit: I did get a plumber in to tell me what the story was with removing the back boiler, he's the one that told me it's a direct system. There's no header tank, or vent, for the circuit to the back boiler.

[onesoma]

Cheers tube, there's no coil in there though - the old direct system is one where the water circulating through the copper back boiler is the same water as circulates through the cylinder and comes out the taps.

[LordEoin]

Because they're brass fittings, a spray of WD40 and it should be easy.

[imark]

Are you planning on reusing the tank? If it's an old one it's probably got thick enough walls to take some welly. If it was the newer ones you could forget it.

[CB_Phil]

use penetrating fluid, spray it on, leave it for a bit (hour or so) & give it a shot, if its still too hard, spray again & leave for longer. Use the proper tools too so you dont risk slipping & rounding the edges

[onesoma]

Yeah, need to keep the cylinder.

Just not sure if it's safer (less chance of leaks in the long term like) to cut pipes and fit compression end cap, or to disconnect what's there and screw on some sort of blanking cap.

[CB_Phil]

Best would be,if you dont plan to use this pipe/hole again,would be to plug it & get the plug Braised to ensure strength & reduce the risk of a leak