Are you using a wired or wireless connection? What Homehub do you have.
Use this speed tester then carry out the further diagnostics and post back a screen shot all the results including your IP profile for up and down. Do this before the speeds normally slow down and again when they have slowed down.
Your IP Profile is not very clear but it looks to be 77.xx Mbps which is very good and would indicate that your line is very good and that DLM is not lowering your speeds. As you did the speed test on a wired connection that would rule out any wireless interference.
Due to this happening only in the evening, and to try and rule things out, have you noticed if it always happens at the same time?
Is the homehub sharing a double electrical socket with anything, such as a TV?
Is the homehub situated near anything that could cause electrical interference?