I have just received an email from DACS in response to why the 28% drop.
I'm sure they won't mind me sharing, there is nothing contentious, and it may answer a few questions.
"We mentioned that you may notice a proportional decrease in your overall royalty payment this year, even though DACS received the majority share, almost 90%, of the monies available.
This is because there were less funds available this year than in 2016 and also, we have received many more claims from artists over the past two year. As 2016 was an exceptional year for the amount available to claim for Payback, the number of claims did not overtly affect the amounts received by artists last year. However, this year's pot of money was a return to monies received in years prior to 2016 and so the number of claims has now started to proportionally affect some of the payments being made.
As you are aware, we also had to introduce new changes to this year's application process. This year, 90% of funds were available through the traditional application process where everyone who applies and supplies their samples would receive a payment based on their claim. For the new publication history pot for the remaining 10% of funds available, artists here would only receive a payment based on the number of matches of their publication history that was matched to the Copyright Licensing Agency's list of photocopied or scanned publications. If you would like more information about the changes to Payback, please visit our FAQs.
Regarding the 10% matches, I can confirm that you did indeed match both book and magazine publications with regards to the 10% pot – 4 book publications with 4 images in total and 2 magazine publications with 3 images in total. This royalty is included in your payment and the breakdown of this royalty payment will be included in your postal statement, which should be received to your address this week."
So without the extra ISBN info my claim would have been even less, so it was worth the effort... well kind of.