Well I've done a bit more research, via Google, our AV dept., and standing on a table with a torch...
From looking at pics/dimensions I'm fairly sure they're GRF's (not autograph though, sadly). The drivers are still there (well some drivers are there - can't be sure they're the originals at this stage I guess but there's no sign they've been tampered with). According to AV they were working up until 3 or 4 years ago when they installed a new system. They left these in place because it was easier than getting them down. They're very dusty/grimy and have accumulated some paint around the edges over the years. They have a red metal Tannoy badge in the upper right corner of the grill - not sure if that will help to date them?
The good news is that in principle I can have them. Getting them off the wall could be a considerable technical challenge however as they're about 9' off the ground and secured to the wall with big feck-off metal plates and screws. I may have to wait until Xmas when some scaffolding can be erected...
I'm tempted to turn up at the weekend with a mate and a ladder, but I guess there's a considerable danger of damaging both ourselves and the cabs. (Does anyone know what they weigh?) Waiting til Xmas would give me some time to persuade SWMBO that I can keep them at least for a while.