I put a link to your post on a Facebook group called Motorhome annd Campervans UK, here are some of the replies, see if they help. One guy also posted a set of diagrammes.
Sean Birch has it got a sargent fuse box fitted ? ours is behind the drivers seat,,,,it has a fuse in there ....some people remove it themselves to stop the step going in and out every time to unlock or lock your van,,,it might have blown....
Richard Bayliss I had a problem similar to this and I sent the Sargent control box, not fuse back to them and they changed a capitor (I think I rember right) which controlled the step. It was the unit in the wardrobe which housed the 240v RCD and the switches for the Hot water etc. If you speak to Sargent technical they might be able to help. 01482 678981.
Steve Jones There will most likely be 2 fuses too; one on the manual feed and one on the auto feed