Since posting my question, I have interrogated Google and it seems the beam axle has a degree of flex. The axle is designed to have a small amount of movement built in. Reading on and it seems that the racing boys do use the rear anti roll bar on the Jazz BUT it appears that fitting an anti roll bar to the rear axle can have a detrimental effect on the welds etc as it interferes with the designed flex.
Another has written that body problems have occurred. Bearing in mind these are people that thrash the living daylights out of their cars.
It was also said that coil overs would produce a better effect without any possibility of structural damage.
Don't shoot the messenger I just read this, how accurate it is I know not.
Incidentally, good old Wikipedia reckons these 'torsion' beam axles save the manufacturer about £100 and also time in assembly. If you watch the latest Citroen ad, when they fly out of the car park, the inside rear wheel lifts off the ground, something that Jezza complained of when testing a VW Golf GT but it seems VW now fit proper rear suspension on the latest Golf GT.