Thanks for the replies! Took me a while to spot they were SL dynamos.

Shame Thorn doesn't offer the option on any of there forks.

I have two bikes for "day rides and lightweight touring". Both these bikes are faster than my Raven, which is a drop-bar "heavy build" with front and rear racks and rather similar to the Nomad, I use my Raven for camping or utility loads and when I know I will be using paths and tracks. 

One "lightweight" is a Raven Sport Tour, which is basically an older version of the Mercury but with 26" wheels. Mine has drops, Rohloff, and lightweight rims. After trying narrower tyres I settled on 42 mm Marathon Supreme tyres which are a good comfort/performance compromise for me and allow mild off-tarmac riding. I started out with just a saddlebag, but added a rear rack for more versatility a few years ago.

The other is my 1977 Woodrup with Reynolds 531 tubing. Geometry is a bit more relaxed than an Audax bike, with space for 32 mm tyres and mudguards. I have Continental Grand Prix 5000 tyres in the 32 mm width, these have very low rolling resistance but are best used on good road surfaces. Derailleurs until I got fed up with cleaning them a few years ago, I now have a 5-speed hub gear. Despite the hub gear, this is still my fastest bike on rides that aren't too hilly, but I don't use it all that often. For a derailleur bike, the Thorn equivalent would probably be a second-hand Club Tour.

Here in France similar second-hand derailleur bikes come up from time to time for 50-100 euros, but Audax-type bikes and outright racing bikes are much more common. I am currently renovating an old Dawes ladies-frame lightweight with mudguard clearance limited to 28 mm tyres - too narrow to be comfortable for me but perhaps OK for a lighter (or younger) rider.

The current range of Thorn solo frames is the Nomad and the Mercury 40.  With mix and match forks, wheel sizes and components to build into a variety of bikes.  There's also some of the Mercury Mk3 framesets still available in some sizes and colours.
I like my Mercury, it zips along at a similar pace to my old Audax bike, with a touch more comfort and slightly more stable steering.  Despite looking quite similar, it rides very different to my Nomad.  I'd say it is at the livelier end of touring bikes, it suits me and is without a doubt my favorite bike, though the Nomad is my most ridden. I haven't ridden a 40 to know the differences. 
Is a sports tourer what you're looking for? Or a different build?  Do you want it to have a Rohloff (The dropouts on both current models take Rohloff or derailleur).  Straight bars or drops?  For a sport tourer with straight bar geometry and Rohloff fittings, the Mercury doesn't have that many competitors. Similar bikes made for drops and derailleurs are a lot more common.
Alternatively, a secondhand Audax may suit you, if you can live with the restrictions of caliper brakes and 28mm tyres. They have fallen out of fashion, to the extent that there's some real bargains available. 
 

£450

We camped at Horgabost on the north side of Harris and cycled anti clockwise round the Island. Somewhere on the S.E. corner there was a sign stating "not The Golden Road" and further up the coast there was another stating "The Golden Road". I can't say I noticed much difference, it was all beautiful. There must be an historical reason for the name so I've ordered The Soap Man book to educate myself! We did see a monument to local folk who had stood up for themselves but didn't stop to glean more information, perhaps a similar era? Hopefully the book will reveal all.