A few tweaks later, and the second incline is a bit better than it was.
I moved the start of the incline back to F1, and adjusted the height of the supports all the way along the trackbed. With a longer incline I have been able to soften the gradient and (so I hope) make it easier for trains to get up the slope.
In the course of making these changes I decided to test the slope with real trains. (Only afterwards did I realise that this was the first time I had run a train in the loft!) The revised slope is fine for locos that are running well: I recently acquired an Airfix class 25 from a friend and this pulls a train up the slope quite happily. But an older Lima class 33 struggles; the lesson is that I need my locos to be running as smoothly as possible.