Hi all, thanks for advice and pictures. I did have the plate holding the hinge but it was fitted the other way round to the diagram. The bolts were in a recess, behind the plastic panel behind seats and I had to spend half an hour removing black mastic before the bolts were revealed! Now all fixed and can take the car back on the road.
You might also want to try SELOC which is the UK Lotus forum - loads of enthusiastic members and events plus technical advice on all Elises (this forum mostly deals with the Fed Elise - 111R and newer).