Thanks everyone for the advice. Last night I liberally applied Vaseline to the side screen fixings and took the car for a spin.
It was better - but after 10 minutes the scritching sound crept back. So I pulled over and tightened the side screens again with the tommy bar. I already thought I had tightened them as much as they could go ! After that the noise stayed away - I drove for 20 minutes over some pretty rough roads so I am hopeful that I have discovered the problem and a solution.
The much quieter cabin meant that I discovered other noises that had been masked by the dreaded scritching sound
But these are not as annoying and might be caused by the bonnet tapes so I will apply Vaseline there as well. In fact, I might as well keep a tub of Vaseline in my glove pocket - it seems to be an essential piece of kit for the Morgan driver.
