Thank-you for your suggestions. I will use a combination of all of them

.
I have decided on a 450mm square tile of 20mm XPS foam.
I have mounted all the pieces on 1.5mm thick plasticard or on Perry plastic figure bases (which are also 1.5mm thick).
I arranged the pieces so that I can make a workshop scene for the final photo.
I drew round each base and then cut it out with a kitchen knife.
With the Proxxon Thermocut, I shaved 1.5mm off the blocks and then glued them back in place so that I have a recess for each base.
Unfortunately I was not accurate enough and many were too wide. I needed quite a lot of filling to get them to look neat.
This base has been covered with sand and undercoated with brown paint.
The roads are wood filler which I tried to imprint with wheel tracks.
This was a complete failure so I will try something else - maybe Milliput.
