Gary - the images are generated ok by weewx - big and small. The problem was the serving of the images by the webserver. Whilst looking around for a solution Edge has magically decided to refresh its cache so now I do not have a problem anyway. This looks like an issue with the caching that Edge does locally rather than the server. In fact, using explorer worked just fine for big and small - but anyway, as i say, now even Edge is working fine for big and small.
Oh well, such is life - this just goes into the "life's mysteries" category for now!!!
Andrew