My Sherpa has a 54cm top tube and I'd like to grab an extra inch or two to placate my ageing back . Accepting there is obviously a limit to what is possible is it better to change the stem, seat post, combination of the two or forget the whole idea. I currently ride drops but switching to straights of some sort is something I can consider.
Your thoughts would be appreciated.
A longer top tube will simply bend your back more acutely, other things being equal.
What you want is an effectively shorter top tube on the same wheelbase, with an effectively shallower angle to the seat and steerer tubes.
Doing the job right might be too much for a Thorn owner, who comes with a certain mental baggage about what a proper bike looks like, and how one rides it. I was once in your position, a wrecked back, in and out of the physio three seasons in the year, but I didn't have a roadie or tourer background, so my path to a bike that gives me not the slightest twinge of discomfort was pretty short even if it took a few years and three progressively better bikes to arrive at a full solution; at least I didn't have any baggage (besides ignorance) to hold me back.
To try out a better position for your back on your existing bike, do this temporarily (you can buy permanent components later, when you understand better what you're doing and how far you want/need to go):
1. Move the seat back on the rails as far as it will go. Adjust the height to touch the pedals with slightly bent knees. If you can borrow a layback seat post, use that.
2. Turn your stem around so that it points backwards. Flip the stem upside down if that will give you more height. Refit the handlebars.
2A. Alternatively, borrow North Road (old-fashioned commuter type sweptback bars; the On One type currently popular are called Mary bars, I believe), and just flip the stem for extra height but without reversing it.
3. Ride the bike. Effectively you've now turned a semi-sporting to sporting bike into one where you're using more relaxed, older angles. If you like the result, once you get over the strangeness of the (temporary) appearance buy the following:
Wider saddle than you would use with drops. Brooks B67 types are good. I use a Brooks B73 with three coil springs but you may be used to a more controlled saddle. If you're a big guy, go all the way up to the Brooks B190. The more upright you sit, the wider your saddle should be.
Short adjustable stem. As Dan explains, you want to bring the handlebars closer to you and higher at the same time.
North Road bars or moustache bars or at least angled bars (like the Thorn ergo bars, though in my opinion they don't go far enough). The purpose is to bring the handles closer to you and to angle them more ergonomically. The bars should be the width of your shoulders, quite a bit wider than you may be used to with drops.
You may, as Dan also says, need a fork with an uncut steerer and some spacers, or a steerer extender; any bike shop will be able to get you an extender from Kalloy or BBB, or try the internet.
Kalloy makes a good North Road bar in alloy and steel (I have both). They also make a crude toolless adjustable stem but it is priced right. The reason for buying cheap parts first is that you never get it right the first time; I circled in on my complete solution.
Straightening you up will inevitably slow you down; you'll have to adjust your attitude at the same time as you adjust your bike, but at least the adjustment will keep you riding rather than giving up so as to be free of pain.