Many thanks for your quick replies!!
I've had a go (as I have to send the job off really quickly)! I selected squares individually, used the median filter & lowered the opacity to around 30-50% then used levels to brighten them up again. Added some noise & cloned here & there. Definately a tricky one & unfortunate I don't have much time to spend on it! I look forward to any more thoughts & advice from you guys!
Stacey