I notice you're in the UK, so it will be faster, and perhaps easier and cheaper, to find a local shop to clean the injectors. In the U.S., some mechanics have the equipment to do this, or there may be a local service like our Mr. Injector. I've just sent mine off to him, at 106,000 miles, for probably their first cleaning. Looking forward to the results.
Otherwise, I like Chevron Techron fuel system cleaner a couple of times a year. Your local brands may vary.
No opinion on Marvel Mystery Oil or other cleaners.
Edit: I see you did change your fuel filter, good.