What would I use to fix this?

I'm not even sure what to call this. It's a 2' high wall thingy in my back garden. A couple of the pillars have come loose and the top bit has come off.
You can see a photo here:
http://www.flickr.com/photos/7204575@N04/2440648953 /
The pillars and the top bit are in good order, they just need "sticking" back together.
My question is, what would I fix this with? Just cement or is there something a bit more specific I should use?
Does the residue of the previous fixing material need hacked off first?
Any advice gratefully received.
- Robin
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In an earlier contribution to this discussion,

It's called a balustrade - see http://en.wikipedia.org/wiki/Balustrade
Carefully clean off the old mortar, and re-assemble using new mortar.
--
Cheers,
Roger
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Robin wrote:

Car body filler. Done it many times. Make sure the surfaces are clean and dust free though.
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Stuart Noble wrote:

ALMOST the same, but epoxy 'mortar' is slightly cheaper and a slightly better color.
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