To answer the Cuprinol question. I visited Brewers this morning and picked up the Sadolin Product and Colour Guide. I also sought advice on whether to use the basic wood preservative as a breathable primer or the 5 star wood preservative. The major difference is that the Cuprinol 5 star Complete Wood Treatment kills woodworm, prevents re-infestation and also wet rot. This certainly would be the choice for an existing un stained and un painted shed on our property, but might be a "luxury" for new rough sawn timber buildings. However, as the log cabin material is not rough sawn, it would be advisable to use the 5 star treatment.
So, for the existing shed I have bought the 5 star wood preservative and will see how I get on with it. I should add that the garden has been neglected for years with no obvious ongoing maintenance of the shed. It is, however still standing and water tight.
The plan is then to match the exterior opaque but breathable Sadolin Superdec Satin with an existing choice for the brickwork on the main residence. This is a Valspar paint which I have used with great success in both Texas and North Wales.
I hope this makes sense!