Hi Mike,
if it's the smaller of the two then it's a 4.5mm seal you need.
The big pipe may have either a 6.5mm rubber seal or a brake-type flare and if the latter then in the bottom of the threaded bore in the pressure regulator is a round metal seal. It's one you can be unaware even exists and forms a seat for the flare. These occasionally go missing if the regulator has been disturbed at any time and it's fallen out unnoticed. Pleiades can do you a new one.
You may find the pipe unions just need a gentle nip-up but not too tight.
I trust it's not the regulator itself leaking. If so then it's off to Pleiades to have the O ring in the bracketed end replaced. It's not really a DIY job that as you need a press to compress the big spring to enable the C clip to be released so that the end bung with the O ring can be removed....
Dripping regulator
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