Pruning a fig tree?

Not sure if this stands for Britain, but here in the rather chilly (at the moment) village in the mountains of northern Spain there are two fig trees. One is on the land of a rather lazy guy from another village and he does virtually nothing to his tree - just lets the birds peck away at the fruit and the figs are getting smaller each year with the 20ft tree looking lank and pale-leaved. The other is in the garden of a hard-working man who cares for his tree by pruning it back hard every two years and this year the fruit is wonderful and the 15 ft tree looks healthy and bushy. I leave you to decide which route to take with your tree!

If the roots are restrained & you have removed the supporting wall it is unlikely the plant is stable.
My best suggestion is that you relocate the plant to another wall (Pref south facing) when it is dormant.
(You could take out an insurance policy by layering & propogating spare plants during the growing season)

With a mature plant like your own you are most unlikely to harm your plant no matter how hard you cut it back - it will return with new shoots next year from the stump. You can easily reshape the emerging growth as you like and it will have no problem fruiting again once the branches get bigger.

However, you might wish to move it because of its location. This will be a lot of work and unless you can take out a sizeable chunk of root which is 'surviveable' it's best to treat the operation as a stump removal and restart with a new plant.