Slipping saddle pad!

Sorry to say so, but if it slips back then your saddle most probably doesn't fit (or your girth is not tight enough). I have dressage Roma pad and don't have issues with slipping. When my saddle started slipping last year the reason was very obvious: saddle didn't fit anymore because my horses changes shape quite a bit (call to saddle fitter who did adjustments solved the problem). Instead of looking into different pads (BTW thicker pad won't solve the fit issue) I'd rather call the saddle fitter.

And if you look for non-slip pad here is the thread: Non-slip english pads

I agree with Kitten Val.

I don't have that exact pad, but I use the Poly Pads Classic pads, they are similar to the style that you have with no girth or billet straps. I also use quilted cotton pads with a Mattes sheepskin 1/2 pad on top that doesn't have straps and I have never had any of these pads slip.

As Kitten Val mentioned, check the saddle fit. If your saddle fits well, when you are tacking up, make sure to pull the withers area and spine of the saddle pad up into the gullet of the saddle and that your saddle is sitting in the correct position on your horse's back, then make sure your girth is done up tight enough to hold everything in place. I think the cause may be as simple as not having the girth done up tight enough.

This is why I was interested in pictures of the whole tack ensemble being used

I agree on the saddle possibly not fitting. I use a western back on track pad (I don't have the english one yet) under my regular saddle pad and I've never had it slip yet.

One of these saddle pads would be exactly what you are looking for. Nice and thick which like you said, would not require a half pad. They also do not slip back at all.

You can buy them at Equestrian Life and Fashion

I'll look at those, thanks.
Here's some pictures of my horse tacked up that someone wanted to see. This is the saddle and pad I usually use.

I like those!

I'm pretty sure my saddle fits him well.

You would love those pads. They are really nice. :)