Only took five months to find someone to come and hang them but finally I have got shot of the ugly up'n'over affair and already trying to maximise the additional space that's created. Thanks to all who offered advice and answered my (many) questions.
A Union J2100 5 Lever Mortice Lock with a matching rebate kit. The kit cost almost as much as the lock and the supplier wanted to charge a further £8 (plus VAT!) delivery if I ordered online and used their click and collect system! As someone observed in my other post about locks it was bit of a barsteward to fit. Although, having said that, had the instructions been a wee bit clearer then it needn't have been. My advice to anyone having to fit one would be fit the rebate kit first as that determines where the keyhole needs to go. We didn't do that and had to move the keyhole approx 15mm towards the door's edge, leaving an ugly hole bigger than the supplied escutcheon plates! I've squared off the 'inside' hole and plugged with a piece of offcut with a new keyhole cut in it. That took three attempts to get right, after which I thought of a simpler (and hopefully neater) approach to tackling the external face.
I cannot begin to describe how pleased I am. I've lusted after these doors for AGES and to have them sitting in the studio for five months after finally being in a position to buy them was particularly annoying. With the prospect of a couple of dry days ahead, I spent today sanding down the white surround , taken it back to bare wood so it can be stained same shade as the doors. There seemed to be a lot of movement in the cross rail and after removing the trim to sand it I found out why - there's a 32mm gap between it and the rsj above! I ended up planing and sanding a section of a scaffold board to plug that gap and that's made the whole frame a lot sturdier. And I have to make a sill as the doors when closed are about 25mm in front of where the original concrete floor ends. All good fun and keeping me out of mischief.