I think it's a coaster swing or similar. GVW 750Kgs. I rang everyone and even indespension who told me I needed some imperial size which I bought but didn't fit. In the end they turned out to be M6. I got some of these : Grease Nipple Fitting M6 x 1 Straight (10) - UK Made | eBay
So far so good, the fit perfectly and don't seem to snap like the originals. I think they are just very poor quality as so far all 4 original ones have snapped first time I used them (dealer sent me a couple of replacements that did the same).
Also, the bearing seem to need grease every time you launch as every time I re-grease a fair amount of water comes out the tiny hole in the centre of the bearing cap before grease comes through.
Hope that helps !