It depends on the level of bike maintenance you intend to do. If it's just a few adjustments on a trip of a few weeks, then a multitool might suffice. If it is a major clean and replacement of key parts on a major trek, then a few more separate tools will be needed.

Essentially, start the trip with all parts on the bike in sound condition (ie, new or near-new). That counts for a lot in reducing maintenance.

There are many ways of cleaning a chain, but usually, taking it off the bike is the best. If you don't have a link that can be taken apart (a la KMC or Sram), then you will need a chain breaker. If you have a history of breaking chains, the chain breaker also will come in handy for those repairs.

What sort of headset does the Thorn have? If threaded, you will need something to service the headset bearings... but then, are you on a tour where you need to do that?

I gather bottom bracket bearings are very reliable these days, so you shouldn't need to worry about those. Wheels bearings? Well, again, it depends on whether you are touring or trekking, and the quality of the hubs.

Soooo... I think I would get something like a Topeak or similar quality multitool, check to make sure all the allen keys work on your bike* and go from there. Ensure you have tyre levers, and a patch kit irrespective of how many tubes you carry as spares. You might incorporate a spoke key that fits the nipples on your wheels (they do vary in size between bikes).

I've gotten by on many randonnees and tours with a Topeak Hummer that incorporates a chainbreaker (although I use the KMC missing link) and tyre levers with spoke keys.

On occasions, I have included a 6" shifting spanner/crescent wrench and a couple of small cone spanners, but that was back in the days when I was using lower-qualilty hubs. Topeak has some small ring spanners in some of their tool kits that can cover small fittings such as pannier rack ends.

*I say this because there are odd little things like the allen screws that adjust the tension on canti brakes and V-brakes that sometimes have been an odd size when I have gone to adjust them. It might seem a minor point, but there is nothing more irritating that a rubbing brake block because the tension on the brake arms is uneven.