Finally I came to the conclusion that I would have to stick mdf to the back of the original profiled sheet. Looking at it, I realised that there was a fair amount of bare mdf above the plaster which I couldn't see how to disguise... out with the jigsaw. After some fairly scary air sawing I hacked as much of the offending board away. I caused a fair amount of trauma to the existing landscape and couldn't get the saw in the right place to trim the far end.
Out with the plaster and plastic rock and I bodged and fudged a taller rock face and hid the bit of 2 x 2 timber which formed the back corner.
The new high cliff and Collie Bridge
Blackhill station in the foreground and the new profile
The next step is to get some more mdf , glue the sky paper already in stock for the job to it and then contrive to attach it to the back of the layout. Not sure how to do that bit yet but I'm sure something will occur.