Don't know if this helps but it's a G-G-ish,G/G,D,G progression. (I don't know what chord is made of G,A,D so I just call it G-ish in my head)
When I play it (not as well as Roy, obv) it starts out as G(GBD),G-ish(GAD),G-ish(GAD),G(GBD) then some vamping (starting a G chord an octave higher and then doing an arpeggio down over the three positions of the chord), then the 2nd part/response is G(GBD),D(DF#A),D(DF#A),G(GBD).
I based mine on the Bottom line '75 version which is a little less busy but same concept as Hammersmith. If you know your G scale and the three ways to make a G chord on the piano you will be able to find his little flourishes and make some of your own.