I got in and it didn't break!  I've made a couple of locking bars out of 12mm steel rod that are doing a cracking job of preventing it from collapsing.  These fit into two brackets that I've tacked to the plates on each frame.  This does, however, mean that all there is between me and a trip to the hospital is a couple of tack welds!  But that's fine, as I have complete faith in my welding skills (cough).  I spent the lesson today doing a series of photos of the cockpit at the maximum angle in each direction, and measuring the distances between the ram mounting plates.  This will (hopefully!) allow me work out what the max extension of the rams needs to be, so I can think about actually getting some.  Either way, I'm pretty excited to have been in it without it falling apart!