Stannah 260 cutting out going up but ok down?

I have seen several questions on here re stairlifts only going up a few inches then cutting out. Ours was ok going up but not with any weight on it and fine coming down. As suggested on here, I have replaced the batteries but it still isn't working at all. I tested the batteries I took off and they are actually both at 13.5v (£35 wasted :-( ). The edge switches all seem free. I am wondering if the 'up' switch on the arm may be faulty but not sure how to get into this. Anyone any ideas? thanks