Had one of these years ago and never got to the bottom of it, wpl dw (eg:854549415710) the feed for the top spray arm pops off its pipes at the bottom next to the lower arm. Both arms are clear, if you remove the bottom arm, it stays on, oddly enough (useless fact!) but remove the top arm (with the bottom arm on) it will still pop off. Anyone solved these?
Usual culprit is this http://shop.ukwhitegoods.co.uk/Dishwash … +Igni.html
If you look at the lugs that hold the pipe on you will find it is either missing the top part or it is cracked at the top causing the pipe to jump out during the wash.