A hail of stones greets the two armoured cars as they rumble past the tree trunks, concrete blocks and burning tyres that litter the main road in Thokoza, an impoverished black township a dozen or so miles south-east of Johannesburg. Policemen wearing riot helmets, their rifles primed with rounds of rubber bullets, jump out and for the next hour or so play cat and mouse with demonstrators mainly in their teens and 20s.