basic plumbing question (toilet supply connection)

Hi, apologies for posting a newbie basic plumbing question. My toilet requires several pumps on the handle to flush - pretty sure that means the diaphram in the siphon has gone. Cistern is directly on the pan ("close coupled ?") so it needs a bit of dismantling to get at it.
It sounds like the sort of thing I ought to be able to do myself, but have little plumbing experience. Unfortunately the supply doesn't have it's own isolation valve so I can't leave it too long once I get started. (We do have another toilet so it's not an all-out rush, but can't take the several days I usually like to give myself !) So I just want to know what to expect if/when I start. (I'm the sort of person that stares at a problem for a good long time before actually laying hands on.)
I've uploaded a couple of photos to http://myweb.tiscali.co.uk/daved/diy / Sorry, they're a bit dark.
"supply.jpg" is the supply side: a plastic pipe with outer diameter 21mm (presumably the base of the bit inside the cistern) connects to a metal pipe with outer diameter 16mm. There's a single nut of 24mm at the join. (Plus a plastic nut up by the cistern.) The metal pipe uses soldered connections off the the supply from the cold tank. I assume the junction is just some standard way to connect a plastic pipe to a metal pipe. Should it be relatively easy to open ? Any tricks if it isn't ? There's no obvious way to support the metal pipe as I turn : should the soldered joints be strong enough to withstand what small amount of force I'll be able to apply, or should I try to support it with something ?
What will I need to replace to put it back together ? Compression joints talk about olives and things, but this doesn't look like the sort of compression joints I see pictured online. Or best to just wait to see what's inside.
"overflow.jpg" is the overflow side. The plastic thread from the cistern just connects to a plastic pipe leading outside. I assume that's just a push-fit or something - doesn't need to withstand any water pressure..?
The cistern is attached to the wall as well as the toilet base. Is the wall mounting taking all the weight, or is that just for stability ? I guess once the water's all out it shouldn't weigh too much. (It's 9l capacity I think - china or whatever rather than plastic.)
thanks for reading this far.
Dave
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OK simple ... job first ... fit an isolation valve .. use a Hep20 plastic valve ... once that is fitted .. you can then take as long as you want fixing siphon.
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Hmm - there's no room between metal supply pipe and toilet to add an isolation valve, and I don't really want to go cutting into metal pipes for my first DIY plumbing job.
What I am wondering is whether I can just screw an end-cap onto the supply pipe once I've detached the toilet. Part of a compression-joint end cap looks like it's about the correct diameter to go on there, though I don't know if it would be the same thread.
Dave
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On 24/01/2012 14:50, Dave wrote:

It will be 3/4" BSP. Any plumbers merchant will sell you a blanking plug.
--
Dave - The Medway Handyman www.medwayhandyman.co.uk

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On 24/01/2012 17:59, The Medway Handyman wrote:

Are you *sure*? All toilet cistern inlets I've seen have a 1/2" BSP thread.
Also, I don't know how well a blanking plug would seal on a fitting designed to screw onto the end of a threaded pipe rather than a solid plug. A 15mm compression fitting with one nut and olive removed, and with a short piece of pipe with an end-cap the other side, may be better.
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Cheers,
Roger
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On 24/01/2012 20:59, Roger Mills wrote:

Sorry, 1/2". Grey moment.

It would.
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Only if you get the right compression fitting, not all have standard 1/2 inch BSP thread for the compression nuts. Some (and I have examples here) have a much finer thread.
Mike
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On 25/01/2012 09:07, MuddyMike wrote:

I've certainly found finer threads on some radiator valves, but most ordinary compression connectors - metric or imperial - seem to use 1/2" BSP.
But I like your idea to use an old tap.
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Roger
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