I have just had another stab at this. I'm still getting the same odd
behaviour but looking in the acces_log I see error code 206 for each
image. This error is described as follows:

"206 Partial Content

The server has fulfilled the partial GET request for the resource. The
request MUST have included a Range header field indicating the desired
range, and MAY have included an If-Range header field to make the
request conditional.

I'm really none the wiser... Anyone have any ideas?