That guide does mention the drain in the centre of the scuttle, this was the fault I found.....
I would suggest a hose pipe in the scuttle and check that it is running out from ALL the drains, including the centre one.
At the back of the engine, approx in the centre, run your hand down the sound-proofing on the bulkhead until just above the the heatshield - there is a small hole in the proofing material, poke your finger in there and you will find a little rubbery flappy thingy, give it a twiddle with your finger and it should gush!! Yeh, it does get blocked and is a common problem, a cure is to remove it.