This only occurs (from what I can find) in provia and velvia from iso 200-320 all f stops, using medium hard highlights and medium hard shadows and ONLY in JPEG, raw has it on the camera, but not in silkypix.
Hot off the press, just got a reply from Fuji UK. Dear Paul Till, Thank you for contacting FUJIFILM UK and we apologise for the delay in replying to you. Thank you for the example image of which we have inspected. Please note this is not a fault with the camera, this is a digital artifact known as blooming and this can occur with all digital cameras, regardless of make or model. You can use a smaller f-stop number or ND (Neutral Density) filter to reduce the amount of light, increase the dynamic range to reduce blooming and I would also suggest using the histogram to expose the image, when capturing similar pictures. we apologise for any inconvenience caused.