You tried running a cloth dipped in vinegar along the length of the blade? Usually do it to mine every few months and after 2 years they still don't smear. Takes grease/car shampoo off the rubber blades
It's a good suggestion, which I must confess I didn't try here before binning the old ones. I've tried it a few times in the past and never found it to work so have rather given up on it. I can probably still retrieve them from the wheelie bin if they haven't been bent out of shape by the bags I've dumped on top since, which might be worth a go. There are also a few light (but lengthy) scratches on the surface of the screen, which the replacement blades may be struggling more than other brands to accommodate. I don't even know how long the scratches have been there; perhaps it is they that are causing the existing set to smear. Like I say, at £6.59 for the pair I thought it worth trying a second set on this occasion so I don't form an unfair impression from a dodgy set. I would rather be able to pay that for replacements going forward than closer to £20. If this replacement set smear I will try some new Valeos (which I know worked in the recent past) and if they smear too I will know it is the screen (and I will have enough sets of blades to last until my next car, probably). The screen may have to be replaced shortly anyway, as a 4 or 5" crack has just appeared in the top-right!
"Perhaps you have something oily on screen have you tried cleaning screen with fairy liquid."
Actually; you could be on to something there. I did accidentally get some small splashes of 'Showroom Shine' waterless cleaning solution on there a few weeks back. I wiped it off with a dry cloth but it may well have left some oily/waxy residue. I wonder if that has coincided with the smearing. I had forgotten all about that and your suggestion reminded me.