the 'fall' in sales was noticeably greater for Diesel cars, thus suggesting that uncertainty over these may be an important factor in the reduction.

That would only be true if the fall in diesel sales was offset by an equal or greater rise in other types. The fall in sales is in total indicating that people are just not spending on new cars in general. The timing you mention fits in to the timing of the referendum and it's result giving further indication, if any was required, that people are uncertain of the future.

DtG