add a function to submit options.php

I have a form which does

<form method="post" action="options.php">

.........

<input type="submit" class="button-primary" value="<?php _e( 'Save changes', 'bsp_style_settings' ); ?>" />

I have all the register settings/admin_init’s etc. and it all works fine and saves the data to WP_Options part of the database- great !

All I want to do is add an additional function that is also carried out on submit. So simple call to say

function also_do () {
xxxx
}

so this function also executes when save is pressed.

I’m sure the answer is simple – help anyone?