Thanks for all the replies. On further investigation, the two flanges are still together, it has just come apart at the rear pipe from its flange.
Nissan say it will cost £420 to fit a new rear box but that price may escalate if they break the centre piece flange when removing.
Is it risky to get a welder to try and repair or should i go with Nissan?
Stupidly took it down to Kwik Fit where the local cowboys were umming and erring over numbers - final quote from them was 670 from the cat back, though not sure if this would be OEM...
Other option is Milltek where the guy has quoted 890 fitted, but then i'm not too keen to splash out all this money just yet.
What do you think - you're thoughts are very much appreciated!
Thanks,
Sean