There is nothing better than hearing the answers you want to hear cos I really wanted to put my bike back together tomorrow rather than Monday and it looks like now I can by lightly sanding back etc.
As far as the O ring is concerned, well they got that part of the order right so I'll be replacing that no worries.
So I'm all happy now - YAY , tar all, you've taught me another little trick

BTW, there's a wacker that I work with (I'm at work) who used to be a diesel mechanic and he reckons they used to re-use their copper sealing or crusher washers all the time. What he has suggested I do is clean up the washer as has been suggested using say, wet and dry etc and then put them on a coat hanger and heat them up using a gas torch till they glow an orange colour before dipping them in water. This process called anealing softens what have become hardened washers due to the heat of the motor they were in, so they can again give a proper seal with the banjo bolt.