OK, the problem has finally been solved!
It was the PCV (Positive Crankcase Ventilation) valve after all. Interestingly, nothing showed up on either independent garage or Honda main dealer diagnostic equipment. Honda suggested the PCV and, bingo, it's fixed immediately.
I'd like to add how helpful my local Honda main dealer (Johnson's Oxford) were, especially as they didn't charge me for the diagnostic work and only for the PCV (£61 inc VAT & fitting) which they did while I waited. I also got a replacement drivers door lock barrel fitted whilst there at £104 inc & another spare key which, again, I thought was reasonable.
Thank you very much for your comments and suggestions.