I asked for the brake fuild to be changed and bled to see if this would cure the issue, in case there was trapped air in the system causing the loss of fluid.
I have noticed however that the issue of increased pedal travel (and less fluid in the resevoir) has returned over the last couple of months.. I've just had my annual service at Honda and nothing was picked up on the brakes functioning unusually.
Based on all that, has anyone else had the same issue? Do I just need to get a new master cylinder?