I've read it and I think that I understand it. (I've drawn a diagram to aid my thinking)
As I read it, my example, above, fits your scenario exactly.
'X' (in my example) is the parent of the two boys. If X was still alive you and X would both receive 50%. However, since X is no longer alive, you get 50% and they each get 25%.
However that's 50% (or 25%) of whatever's left after the court has awarded 'reasonable financial provision' to the partner (assuming, of course, that the partner actually applies to the court). So it might be better to go along the route of a 'deed of variation', guaranteeing a certain amount to the partner.
I'm guessing that the 'pro rata' bit might mean a three-way split between the partner, yourself and X (with X's third actually being split between X's sons) but you'd need to investigate further to find out precisely what it means.