Thank you Mark, and I can confirm that the change has worked.
It would be worth knowing (I did try looking for information) that this would have been automatic with a different DMARC setting, but there is an element of "cart before the horse" here. I introduced DMARC and wanted to see where there would be any problems if I did turn on a different disposition (which seems a thoroughly sensible thing to do). By FAR the worst "offender" in terms of reports coming back was groups.io (which is not necessarily surprising considering messages get "broadcast" somewhat). In fact during the recent couple of months with the exception of messages through groups.io the only messages that have been flagged are ones that I would have wanted blocked. I wanted to fix this before I introduced the more restrictive policy as I didn't want messages to be rejected by MTAs that take this into account. At least now I can clean some of the groups.io servers from my SPF too, which I had done to try to fix this!
I can now run as I am for a couple more weeks and check that my reports have dropped right off then introduce something rather more restrictive.