Crumbs, I'm surprised about constraining their roots. Nearly all the ones I've seen are pretty large, either in height, girth or number of trunks, so they must have extremely well developed roots. The local advice here is that if your fig tree isn't fruiting, it needs shocking. It's a poor term but in essence it means chopping back almost all the trunks that might be growing from the base and giving what's left a severe pruning. If that doesn't work, you chop some of the roots back, but this is regarded as drastic surgery. When we moved here, nobody had looked after our fig for donkey's years, but cutting the unwanted trunks (a dozen or so spindly things) did the trick and now it is extremely productive.
Most but not all the trees round my way are growing above terrace walls, so the roots can only develop on three sides, but to cope with the gale force winds here, the trees need to be very secure and must have good roots. I've just had a look at a piece in the Telegraph that talks about containing roots to ensure a crop of fruit. The purpose of this containment is to stop the tree from growing so it channels its efforts towards fruit. You can achieve the same effect by chopping back unwanted growth but keeping well-developed roots to feed the trunk/trunks you allow to grow.
Don't take this amiss, but it seems to me that the Telegraph advice and growing a fig tree in a pot/trough is like growing a somewhat large bonsai. You'd get a far better crop if you could grow a fig tree in a decent site but just keep cutting back any new growth at the base each spring. A once a year task that would save you a lot of effort watering and would give you a very handsome tree and more, bigger and juicier fruit. Whether you could do this with a tree that's been in a pot for 15 years is another matter. So you might do well to try the Telegraph way, but a fig tree needs room to grow and be productive, that much I do know.