Waste Pipe Draining Problem

[EasyGoingPat]

Hi,

Recently I fitted a new deep porcelain sink and had to completely re-plumb the sink and dishwasher. The dishwasher waste pipe follows a fairly conventional arrangement - up and over a standard-height sink side then onto a side-fitting on the sink S-bend (see photo, though note that dishwasher is temporarily disconnected and the hole is plugged).

The problem is that the hole where the waste pipe exits through the wall is about 5cm higher than the point at which the dishwasher hose connects onto the sink S-bend. Predictably, it gurgles when the dishwasher drains. I could live with this but recently the dishwasher has had trouble draining.

Now I do know that the logic of all this perhaps points to some problem inside the dishwasher itself, but before I go ripping the sides of it and changing pumps and the like, I wanted to do what I can to improve the external situation. After all, if the dishwasher has developed a fault, it could be the external plumbing that has caused it.

So I have two question areas:

1. Can anyone recommend a good non-return valve for the dishwasher hose? Where in the hose is the best place to fit this? Is it likely to help?
2. I have seen 'non-gurgle' sink traps? How do these work and are they likely to help?

I would be grateful for any advice anyone can offer.

Kind wishes - Patrick