Cheers for the advice,
Did as suggested by turning the water flow down and it came up a treat, despite the house still being like a building site, piles of sawdust, rubble, painter's splashes on other carpets that weren't there when I surveyed etc. No customer on site (was an early evening job as he moving in next day) for a couple of hours who, when he did turn up, was shocked when I said I was charging extra for the builder's clean.
He'd got the job cheap anyway on the basis it would be clean and empty and I'd be following the cleaners in (didn't accept my quote to clean as customer reckoned he could get a filthy house 3 bed, kitchen, double oven caked in grease , grotty tiled kitchen floor, mouldy windows etc cleaned for £50). On the night of cleaning carpets I suggested he clean the kitchen and bathroom floors of the dust and dirt as it would get trodden into the clean carpets, he asked if that was something I could do for him!
Wasn't happy about extra charge for carpets, but was more than happy the job.
Jon