I can't find an original Hozelock equivalent for this part, so I assume I'm probably trying to plug things together in some way Hozelock never expected. All my fittings are Hozelock and I think the sprinkler is too. In any case, the beauty of 3D printing is that I can fix it simply by mixing together two existing things.
I've added the original things' code here too, just to simplify access.
For some reason, I end up with a small pointless screw in the final STL which isn't removed even though I've tried. Would be happy to have a comment to fix that if someone does know.
I found the I needed a gasket too to stop the flood of water. So I printed off an earlier model I put together (https://www.thingiverse.com/thing:1584431) using TPU.