Total distance of the journey was 1583 miles
The distance of the downgraded leg was 471 miles
471 miles is less than 1500 Km , so the reimbursement due is 30% of the downgraded leg which is 30% of the overall journey

As such , the amount due woulld be 25,000 * 0.3 * 0.3 - 2250 miles

Since you have received 3750 miles already, you have already received mote than the statutory reimbursement. 3750 is 15% of the amount paid - if looking at the 2 sectors being equal, rather than taking distance into account, that is 30% of 1 sector

I don't see that you have anything to chase up on in regards to EC261