You can check the transfer belt for debris.
I appreciate your suggestion but I have already tried cleaning the lenses. It made no difference whatsoever.
Could you please advise what else would cause this problem?
You can try to clean the lens for the affected colors to see if that fixes the issue.
When printing specific colours, light-coloured 'tramlines' appear down the whole left side of the page. I have printed the 'Repeating Defects' pages from the printer menu, and the fault is clearly visible on the blue and green pages, slightly visible on the black page, but not showing at all on the magenta page. Please see the attached photo showing the four pages.
Could someone please tell me what is causing this problem?