I'm really tempted to replace the Ogre with another Thorn, but which one. Realistically I'm not going for long distance overseas tours, mainly day riding with some lightweight cycle camping in the UK.
I'm thinking maybe a Sterling, if I could find one, if not then maybe a Nomad MK2. I'd really appreciate views from those with first hand experience of this type of riding.
If you're not carrying a serious load I reckon a Nomad would be overkill.
My Raven Sport Tour (can be considered as fairly similar to a Mercury) does OK on bridleways and mild off-road with lightweight Marathon Supreme 26" tyres in the 42 mm width. I don't have a rack on this bike, so load is limited to a saddlebag.
If I know in advance that I will be doing a lot of off-road riding I use a bike with fatter tyres, which are more comfortable on uneven surfaces. My Raven Tour copes well on tracks and paths
WITH a substantial touring load, using lightweight Marathon Supreme 26" tyres in the 50 mm width. If planning a long tour with a lot of serious off-road riding I would probably change the tyres for at least Marathon Duremes (still light, but tougher than Supreme), maybe something even tougher, but I would trust the bike itself for everything except serious expedition type touring.
So a second-hand Raven Tour or Raven should be OK, if you don't mind having V-brakes rather than disks. For the Raven I reckon it would be better to have a 531 fork rather than the lighter 853 option.
A Mercury set up as a "gravel" bike with wide tyres (650B) should also be good, but a lot more expensive than a used Raven or Raven Tour.
The Raven Sport Tour is limited to about 42 mm max tyre size with mudguards, so not a good choice as you already have a road-going Mercury.