It appears after testing the bellows at night with a flashlight that the source is there. Quite porous. Usage in bright sun in the last couple of months must have worn the material which was already fading. If someone can suggest a UK fitter of new bellows that would be fantastic and in the meantime would placing a cloth over the top suffice? Thanks.
Toyo 45A mark 2. The bellows were initially fine at first, at least with the 180 lens. Problems occurred with the 300mm which may be down to extending the bellows and thus revealing issues. Usage in bright sun in the last month may have caused faster deterioration.