Be linked to the top bit, or just sticky from lack of maintenance.
Here's the bolts: http://www.tartybikes.co.uk/disc_brake_spares/hope_reservoir_bolt/c110p11213.html - can cut a slot and use screwdriver as said though.
Fit new diaphragm, bleed, spray silicone on pistons, pump brake a little, push pistons back, repeat last 2 steps a few times, then clean up thoroughly and good to go