Thanks Prze and Rick.
... and, Nigel, I've had a look at the WNW pics (should have done this in the first place) and my initial assumption that these joints could be represented by a fine line (perhaps a decal) prove unrealistic as most appear to mark the join of a darker plank with a lighter one.
Hmm. several thoughts - first using a very fine aluminium foil - maybe Bare Metal Foil - as a mask, cutting these little chevrons into the foil then overlaying on planks to apply oils. However I found these ultra-thin foils too delicate to cut accurately. So I resorted to the metal foil from a bottle of Rioja!
This was easier to work with and after several attempts, something approximating to what I wanted was achieved.

To see if the system would work I overlayed on a plank and gently brushed on some darker oil tone.


Well it's not perfect, (and the picture is under a very bright light,) but I think I've got a way forward. I'll practice a bit more and apply a few more joints - bearing in mind that a black line will obscure the middle two planks of this scheme all along the fuselage. also I'll need to tone things down a little, just leaving the darker definition at the actual join. But this will be sufficient for what was intended as a simple OOB exercise. Well it's a tribute to Dave, so worth the work!
Sandy