Bit late to the party here, but I wonder whether one of these would have helped?
https://www.screwfix.com/p/tectite-sprint-brass-push-fit-adapting-female-iron-coupler-15mm-x/965fj
https://www.screwfix.com/p/tectite-sprint-brass-push-fit-adapting-female-iron-coupler-15mm-x/965fj