People, thanks for all your help, but my computer appears to be reacting badly to Opera in other ways, so I'm about to uninstall and reluctantly revert to Chrome. So I won't be following up this printing issue at this stage.
If the other problems continue, once I've tracked them down and sorted them I'll probably come back to Opera.

People, thank you for your interest in this! However I won't have access to the printer for several days now. Should have delayed asking...

I've checked - the driver is up to date.

Windows XP - which I suppose could be the source of the problem.
Printer is HP Officejet 6500. I thought the driver updated automatically.
Websites - several including gmail.

It varies. Empty spaces of differing sizes, where there should be text etc.

About Opera says it's 21.0.1432.67 - hope that makes sense!

I'm having trouble printing from Opera. From at least two web applications with their own Print pages, there have been bits missing from the printout - the layout isn't affected, there are just gaps.