That saddle does look uber comfy, shadowanne!
Really, the depth of seat becomes a problem when you (the rider) feel restricted, or "locked in" to the saddle to the point that it is difficult to move to cue the horse. When the seat is too deep, and the blocks to big, the rider must fight the tack to ride well; not a good thing when the entire point of a dressage saddle is to free up the movement of the horse and facilitate greater clarity of the seat and leg aids. If you're able to get out of it to post the trot, odds are that you're not too restricted. I'd love to see video, though, if you have it.
From your description, it doesn't sound like the seat is too deep for you. However, if you start riding higher-level dressage where precision counts even more, you may find your tack working against you where it didn't before. It's a really subjective thing. If you care to experiment, maybe find a flatter dressage saddle that will fit your horse and borrow it for a ride, and see what the differences are.
Maybe (like so many things) saddle measurement for the rider is different in the UK, but in the US the measurement that tends to give the most accurate match is the thigh length, from kneecap to hip-bone. I have the exact rubric in a book at home, matching inches of femur to seat sizes, but I can't find an online version (someday I will find a way to pack my home "library" into a 4-door Chevy and bring it to college with me...
) Of course, there are a ton of other variables in the mix, and rider comfort is paramount after horse comfort, no matter what the numbers read. Does this Saddle make my Butt look Big? Saddle Fit for Riders EQUINE Ink