Thanks Roger.
I have attached views of the rear of the machine and the top panel (2 views looking down). There are five obvious screws in the rear panel (top left, top right, mid right, bottom right and a more obscure one below the carrying handle and near the lamp. I have successfully removed these screws with an allen key. In the top panel there are just two obvious screws, situated at either end and just behind the main thread spool rod. They are slightly larger than the five back screws and also removed with a larger allen key.
I can't find any more screws to remove. I had expected the back panel at least to come away at this point but apart from some movement around the needle end of the machine, the panels are still attached to each other. I guess that there are concealed plastic clips but I'm reluctant to exert too much effort in case I inadvertently break something important. There are also several screws in the base of the machine but since the dealer said only that I would need to remove the back panel, I don't want to fiddle with the base screws without knowing exactly what they are holding together.
If any of these images look familiar I would appreciate any thoughts that you might have. Incidentally, I tried to get hold of a service manual for this model but they are as rare as hens teeth.