Well after procrastinating for a couple of weeks I bit the bullet and got the base done.
Step 1 take a cd sleeve, not a hard plastic but sort of flexible vinyl with texture.
Step 2 mark the joints of the concrete with a scalpel, then run the blade at 45 along the groove and scrape out the excess. Glue to the base with some weight on the top.
Step 3 paint base and concrete then keeep applying colour till the texture looks right.
A lovely little (1:76) model. I don't know why, but when i was about 19 I threw my whole collection of these away. Madness I tell you. This is one i picked up in about 2001 when revell released them in a box set with german infantry marked as 1:72. The infantry where 1:72 but these ex matchbox models are definitely smaller at 1:76.