Repair help - broken leg  

if possible
couldn't you try and find a threaded insert (i.e a nut) to put into the wood to take the leg screw?
(does that make sense?)

it's got one, attached to the leg.
Just need to unthread it.
I was thinking of maybe using a plate with screw thread (large t-nut?) as an alternative option

Ah
didn't see that.
Yes, that could work!
I had to do that with one of my Hvidt sofa legs that had a similar problem

.
I still can't see the nut - from the pics it looks like its screws directly into the carcass, ie into wood, with glue to help it along - wrong? Incidentally sizing your pics to more web friendly size would help, 600 or so as the max dimension would be good...

the nut is still screwed into the leg
The problem is that when I insert the nut back into the wood it's loose, as the wood has started to split. So there's no point in re-gluing it back in place.
To add: the nut is also threaded, so it does look like the screw.

...You need
an Eames Leg Splint for that broken leg, sorry I couldnt resist!

I'd first get the threaded insert
off the hangar bolt in the top of the leg and consider epoxying it back into it's hole. Be sure to use a quality "24-hour" epoxy product and clean out the hole and the insert thoroughly with cotton swabs and alcohol beforehand. Wet the inside walls of the hole thoroughly with a thin layer of epoxy first, then spread the glue onto the outside threads of the insert, gently slide it into place, and don't touch it for a few days. Be careful not to glob any on the inside threads! You can also find a short bolt that will fit the insert, coat the threads with a little petroleum jelly, thread it in there and leave it in place when you do this to prevent the inside threads from getting fouled If you're not confident, but you'd have to clean it out afterward with white spirits and be very careful not to contaminate the surfaces you want to stick.
The frame member looks to be of softwood and compressed/deformed where the the circumference of the top of the leg makes contact. If the top of the leg is flat and not concave, the mating bearing surface of the frame needs to be flat, too. You can do this by filling the indented area with thickened epoxy.
I don't see any splitting of the frame member, but, sometimes, you can pry splits open just a bit and work some glue in there with a thin spatula or a toothpick and clamp 'til cured.

Thanks, tktoo
I was waiting for you in anticipation!
The splits in the wood don't show, as they are mostly horizontal so the don't show in the photo looking down.
So, basically don't touch the wood and apply a tough epoxy.
I was worried this wouldn't be tough enough, but it seems fairly straight forward.
Thanks tktoo

The joint seemed destined to fail.
Once the leg joint, for one reason or another, became unstable, the leverage of lateral forces deformed the softwood frame and pried out the threaded insert.
I've made exactly this kind of repair to the center leg of a sofa of my own and it has held up to minimal abuse for well on a dozen years now.
I'm a big fan of West System epoxies, but I'm not sure they're redily available to you over there. Their website is a great resource for anyone using epoxies, though.
http://www.westsystem.com/ss/how-to-use/

Thanks again.
I went for the one below, it seems pretty strong (extreme strength up to 100kg/cm):
http://www.my-tool-shed.co.uk/p24211/Unibond_Repair_Wood_952567/product_...

I can't recommend that product.
Look for a long-cure, high-strength product that's unthickened. If you need to thicken it, you can add fillers like colloidal silica or even whiting (calcium carbonate, not fish).
The main problem with using epoxies for repairs is that they are not practically reversible by any other than mechanical means. If you screw it up, it'll only make things worse.
Got a boat supply or chandlery nearby? Those places often have a good selection of quality epoxy products and advice to offer.

Arthur Beale
at the top of Neal St./Shaftesbury Avenue should have that.

haha!
Top of my road, turn left.
Perfect.

the only thing they had
was this (see link below):
Looks good, but i'm not sure which is best now.
http://www.loctite.co.uk/cps/rde/xchg/henkel_uke/hs.xsl/fullproductlist-...