15 stone isn't too big to do vertical work. =) I'm 190 pounds (13.5 stone) of cuvaliciousness and I just got back from doing Golondrinas which is 1200-ft single pitch! I do squeeze box to 8.5 inches, which is where my hips get stuck. Can't squish bone! lol.
Harness, depending on your weight distribution especially, may prove problematic more than most, however (and climbing as well).
Together with a good support team (we specialize in long rope rappel techniques) did some customization of harness type and also completely re-invented a climbing system is what it took to get me doing anything over 400-ft.
Here is my Bungee and a Half system. I do enjoy it for technical as well, as unlike the classic double bungee used here in long climbs, pop off the chest roller and foot ascender and it's a Texas system which works very well for technical rigging (rebelays and the like).
https://www.youtube.com/watch?v=g_QFdZLvnok
A system perhaps more suited for your minimalist types is a variation of the classic Single Bungee Ropewalker, with a home made chest roller and it uses as SBR systems a pantin on the foot rather than a bungeed ascender. Again, it has the same converting benefits of going from ease and fast climbing, to making quick work of technical rigging.
https://www.youtube.com/watch?v=T8MqJnmU8H8
A variable often ignored in seat harnesses is the rise, which is the distance from the leg loops to the waist band. Caving harnesses classically have "no rise" or "low rise", the thought being to keep the croll low allows better strokes on a frog system. While this works brilliantly for those with a frogger body type (tall, lanky), it doesn't work so well for the rest of us. And actually, with prettymuch any other type of system other than a frog, doesn't matter at all as you don't have the croll sitting on the halfround mallion anyway. Either way, I frog much BETTER with a high rise seat harness as it puts my center of gravity into the harness itself, rather than on top of it. If your center of gravity isn't held in your harness, you will fall backward and have a tough time of things. With fine tuning adjustment, I actually get better strokes with a high rise because I can actually stand up rather than fall over backwards trying to stand! And worst case, in the end if you loose a bit of stroke with the higher rise bcause of short arm to body ratio or something, it may still be worth it, as the effort to stand up and advance up the rope may be much less, so although you loose a bit of distance per stroke, you can still climb farther due to the less energy required.
https://www.youtube.com/watch?v=yZuM2n_SwTw
I'm not sure if any of these thoughts help with the troubles you are having, but hopefully they do! =)
