Which bolts do I need?

OK so in the garage I have a pair of R1200GS handguards for my F800GS. To fit them I need a pair of bar end weights, bolts and large 'washers' that hold the handguards in place. I nipped in to my BMW dealer a while back to see if I could get these and they asked what size bolts was needed. I have no idea which size bolts are already in the bars of the F800GS and clearly neither do they!

Candidly I'd rather get 2nd hand parts if available but am still set with the 'which size bolts' question. Anyone happen to know?