I am also looking for sliding door wardrobes at present. I visited Robeplan's showroom last week and their quality seems good. I haven't got a quote yet so I can't testify to their pricing. They will charge you per metre inclusive of carcasses no matter what your internal layout is (within reason, I presume). It costs a good bit extra if you want them to panel out the back of the wardrobe and they also install a higher plinth with this option. It is cheaper to use your existing wall as the back wall to the wardrobe. They will also quote you for installation of the sliding doors only if you want to get your own carcass. I will be going with this option for one of our rooms downstairs to create a utility area to house the washer, dryer and cleaning equipment.
Another place that I will be looking is:
[broken link removed]