You're correct on the plugs, the whiter ones were further away..
I wasn't planning to change the oil as I didn't know it was meant to be dropped but I have now drained from the oil tank and changed the filter for a nice red purolator.
I estimate approx 8 liters has come out now but I'll know for sure when I empty it into a measuring bottle. I'll refill with new oil and run for a few weeks and then change again to get a full flush.
The rocker shafts were all fine, I checked after you mentioned and all is good. I've now completed the job of checking the clearances and buttoned the engine back up. Only 3 adjustments were needed, 2 too loose and 1 too tight.
Just waiting on some new tin ware and plugs then I can get it back together!
Thanks for your help on this.
A straight forward enough job, just takes some time, mainly the strip down and reassembly!