Battery Gradual Discharge

Hi, can anyone tell me why we're seeing this gradual drop off in battery charge throughout the day and is there any way to prevent it? We have the system configured to charge when we have a cheap rate in the early hours and only get used starting at the peak rate, which is between 4 and 7pm.

It's getting us through the peak period fine, even with electric oven usage, but would be even better if it supplied us more into the evening as well, which it easily would if it didn't lose so much during the day.

The batteries are in the loft in the space shown by the 2nd picture.