Risky , skilled and experienced job, root trimming - I'm a professional but I don't consider myself sufficiently experienced to carry out root pruning.. so up to you whether you want to take a risk or not. Tugbrethil might have a more useful answer...

I've done it before, but it is risky, and the loss of a year's flowers is almost a given.
First, wait until the weather is cooling down, just as if you were doing a normal planting. Then, a day after watering it, slip it gently out of the pot. If it refuses to come out fairly easily, or the pot is narrower at the top than in the middle, you may have to break the pot to remove it. Cut narrow, vertical strips, 5 cm deep, down the root ball at the 12, 3, 6, and 9 positions. The goal is to remove 1/4 to 1/3 of the circumference of the root ball. Repot with fresh compost in the furrows, and IMMEDIATELY selectively prune the mass of the bush to remove 1/4 to 1/3 of the foliage. Use your favorite root stimulator in the first watering, and keep it shaded and well watered for the next month or so, until new roots grow. Then keep your fingers crossed--admittedly, hard to cook and pay bills that way! :)

Thanks. So better to place straight into garden and hope for best. Luckily it wasn't expensive (supermarket stock which had wrong flower colour label, suppose to be white but was pink) but may try the root pruning as an experiment. Nothing ventured, nothing gained!