Sorry, Wally, there is no practical way, except to dig it up, prune it back, and plant it in a big tub, preferably in the fall or early spring. The main hazard is that the roots can heave pavement, push over walls, or crack house foundations if the trunk is less than 5 meters from them. If it already has plenty of room, then no worries!

Don't worry Wally, we have NEVER gone in for this root restriction nonsense and our fig trees are healthy and give great crops.
Just keep your tree pruned well and restricted to half a dozen or so main branches. Cut out any growth from the base and the suckers which form.
Unless you have very fertile soil there should be no problem.
Fig trees are grown commonly next to walls and houses and I think the threat to foundations etc is much exaggerated.
Keep an eye on how it grows and if it does become too vigorous, you might need to keep an eye on where the roots are going.

Thank you so much for answers. I think I will just keep it trimmed, it is an espalier form so that should be easy. Thanks again. Fresh figs straight off the tree are so sweet and beautiful that when picking them not many get back to the house!

Maybe I'm just worried because the can get 10 m tall and wide, here! Probably much different in the UK.