How to make apple chunks in apple pie stay intact?
The only trick I know of is to use a different variety of apple-- some will turn to complete mush, while others stay firm.  Unfortunately, unlike potatoes, they don't tend to be marked at the super market as to which variety they are.

Now, I'm not familiar with Bramley, but from what I've found, it's compared to Granny Smith, which tends to hold up well in pies.

I'd recommend blind baking your crust, if you aren't already, so the filling isn't in there as long, and possibly going with a thicker slice of apple when you're cooking, in hopes they won't break down as quickly.

update : in checking CookWise, there's a comment in the apple pie recipe that "briefly sautéing and poaching apples softens them slighly before the sugar, which prevents cells falling apart and preserves texture, is added"