Renault Master / Vauxhall Movano - Panel van sliding side/ loading door problem. - roofie71

I have a 58 reg (jan 2009) Movano van on which the side door is sticking. The issue seems to be that the middle roller tilts too far when the door is opened until it goes 'the other way'. Obviously it has to tilt one way once the door comes out, but I think then it should remain straight to slide along. If I keep some weight on this side of the door as I open it then it helps, but obviously this is difficult when opening it from the inside as you need to pull that end of the door, which dissappears as it opens. There seems to be a fair bit of play in the top and bottom runners and I'm wondering if there is some adjustment that can be done there, or is a new centre roller needed, and if so what sort do I need. I have seen a couple of different types of central / middle roller for the van on ebay and not sure which part i need. many thanks in advance for any help yo can give.