Hugh It's a real bugger when these things happen. When mine was found it took about ten minutes before it was confirmed that it was going back to the factory to be sorted. Sounds like your chassis has gone in a different place from mine but that whole area appears to be highly stressed and it might be a bit of a toss up where a crack will start to propagate from - mine was from one of the top wishbone rear mount bracket welds. Some of the info that has been posted is not quite right, sorry Dan but the two chassis pictures are just that; two separate chassis, not one before and after welding in of the cross bracing. I am aware of three variations of bracing at the front ends of the chassis; 2012 bolt in cross brace only 2013 stiffener plates welded in plus bolt in cross brace 2014 welded in cross bracing only I'm not going to state any specific dates but I can say that in November 2012 my car went into build, in November 2013 it got rebuilt with a new chassis (which incidentally also required a new pedal box I believe to make it fit.) In January 2014 the 2014 chassis cars were definitely being built. There are other changes in the various chassis only a very few of which I am remotely aware of.
Calum,you are correct about the before and after picture. Clearly two different builds. But it is a early frame without a weld in or bolt in brace and a late frame with the welded in brace.
In your first picture you can see rust on the bottom area and then clean steel around the top. So in your case it was probably cracked for some time before it failed and separated completely.
All I can say is that there is plenty of force on the upper wishbone horizontal frame tubes.
