Excuse me for jumping on the old thread (I didn't have B-Max then , can you tell us about those Bars, (producer and model)?
Are longitudinal bars required for this?
Called Ford dealer, they told me that I need longitudinal bars, and the price is about 500 Euros (plus the price of transverse bars, paws etc) OUCH
As you probably know, Thule has nothing to offer, I guess that is because sliding doors are not compatible with standard Thule roof-gripping paws.
I have those and yes you do need longitudinal roof bars for the cross bars to clip onto. I had them fitted when the car was supplied.
The 'paws' as you call them actually have protrusions which fit into corresponding impressions in the roof bars. This stops them moving backwards or forwards. The tension in the fixing bolt therefore only needs to be very slight - almost finger tight.
Only Ford do both. However, Thule boxes and other makes probably also fit the cross bars.