There are quite a few differences between the sets of Spooks shown on this site and the following variations in the figures have been identified so far:

It would be interesting to see pictures from other collections, as it seems quite possible there are more versions waiting to be discovered.

Kim has brought to my attention a book about the plastic figures made by Herald and Britains during the 50s and 60s. It describes in detail the manufacturing process, some of which may be applicable to the production of the Spooks. It seems that it was usual for several moulds to be made for each figure, to speed up production. These would be made from the same master figure, with fine details being added to each separately. Hence, there would be subtle differences between the figures made from them. So, perhaps we should indeed expect to see differences between the sets of Spooks. However, some of these differences seem to be in the large details, indicating that the different moulds for each Spook were made from scratch. A note I had from Kim confirms that this may have been standard practice with 'flat' figures:

I've asked the editor of Plastic Warrior if he knows anything about the actual cost of cutting dies. Whilst actual figures aren't available, the view is that cutting a die wasn't prohibitively expensive until the late seventies. Dies for flat figures like the Spooks would have been even cheaper; apparently, for such, it would have been possible to cut those directly onto the steel without the need for a master model.