My old bosch d/w had an overfill sensor......so guess yours probably does too ....
Why dont you buy a new one online? No salesmen to deal with then
and usually cheaper too 
plus some sites deliver free 
Trouble with white appliances online, IME, is nobody actually ever stocks them, so the "In Stock" bit is normally BS. Which means I get wound up dealing with these rather retards, invariably with no proper phone number to call, cancel the order, try elsewhere, end up in a viscious circle, credit card company blocking the card for suspicious activity, and me going off on one, and... ....BREATHE

I want to, sometimes, be able to go to a shop, pick up the goods, and bring it home. Simples. I don't even mind paying a small premium for that luxury.
When I bought this one, after the old one packed up 3 days before the Christmas break a few years back, off I drove (in the excellent little Rover) to the land of the inbreds (Banbury - apologies to any Banbarians here

), as there were 3 large electrical outlets on same estate (Currys, Comet and whoever took over the Powerhouse one to sell off the stock). Same routine in first 2 shops:
Spotty salesboy: "Can I help"
Me: "Want a dishwasher to take now, as need it before Christmas"
Spots: "Which one"
Me: "Which ones are in stock here"
Spots: "Which ones do you want me to check stock level on, and I'll look"
Me, pointing to 3 or 4 different ones: "These?"
Spots, after disappearing for 5mins: "All in stock" in a cheery voice
Me: "OK, this one"
Off we go to pay
Spots: "When would you like it delivered, first date I can offer is 5th Jan", sounding like he's doing me a favour
Me: "I'll take it away now"
Spots: "No, we have to deliver it"
Me: "Refund that card then, its no good if I can't have it now"
Spots: "Why, whats wrong with delivery on the 5th Jan?"
Mrs TB now having to restrain me
Last shop (Currys):
Youth: "Can I he.."
Me, somewhat dispondant at this point: "I want a dishwasher. Any dishwasher. But you must have it here, and be able to put it in the boot of the car right now"
Youth: "All these are in stock sir"
Me: "Can you check out back, physically, that you have this one", pointing at a random one, "for me to take right now, in my car?"
Youth: "Oh, we don't really hold any stock here, very limited"
Me: "Just look out there and tell me what you have got in for me to put in my car"
Youth, after about 10mins of looking, "We have this one in, pointing at a Hotpoint"
Thats how I ended up with an FDW70