Any conversation around salary has to include their generous pension scheme (better than civil servants), significant expenses that they are allowed to claim for their lifetime in parliament, expensive freebies such as Taylor Swift or access to an Arsenal box, subsidised food and drink, and the opportunities offered for additional salary from outside jobs and lobbying. Oh and if they lose their seat the get a decent pay out, significantly more than statutory redundancy. And for the small number who might have a baby in office, six months full pay, far more than statutory again.
Just focusing on salary when its only part of their actual net income makes it appear meaner than it actually is. They should be forced to stick to statutory requirements as that would incentivize them to improve it quicker rather than yet another exception.