Hi Mark - You can hold off on the new instructions as I think (?) I've sorted the problem...... I decided to go ahead and give the printer a thorough clean and service anyway as it couldn't make things worse and it seems to have done the trick.
Removing and dismantling the Imaging Drum unit was the key - a bit fiddly and you have to be careful but there are several 'how-to' videos available for anyone who wants to try it. (Just 'Google' your printer model number and 'cleaning' for full instructions)
I was amazed at how much waste toner there was in the receptacle, given how little printing has actually been done with this machine. The manufacturers must be laughing all the way to the bank.
The waste had built up so much that the drum was touching it (on the back-side, so to speak) and must have been picking up the waste while still hot from the laser, hence the streaking. Anyway, all test prints have come up clean and correct so I've marked this as solved but will monitor printing and come back again if necessary.
I'll start a new thread regarding the 'Samsung Easy Printer Manager' as this is a separate issue anyway.
Thanks again for the help - invaluable as always!