Thank you for all the suggestions.
I have tried with some 'walnut colour stain', but although I have walnut planking, the finish with this stain is so dark that the definition of the detailed work has been lost which is very disappointing.
I am now contemplating what to do next. I definitely do not want to paint the rest of the model the same colour and I am regretting using this dark colour and think my mistake was to assume that walnut planks must have walnut stain.
Fortunately I have only used the stain on the the masts and crows nest, so I will probably get away with what I have done.
Any and all advise welcome!