First, many thanks to all for the suggestions. I'm a little wary of using nitromors. I'm also wary of T-cut, which I understand is a coarser abrasive than the Autoglym glass polish I've already tried.
Roger wrote:
What is your principal objection to it, Paul and Will? If it is applied properly, thoroughly and regularly I find it makes for a clean and smear-free windscreen.
I have no objection to anything that works and "wears off" gracefully without creating a hazard. However, Rain-X wears off most ungracefully. I found out about this the hard way recently in the Lakes. The A591 on a rainy night was frightening. Although I could see the road immediately ahead, oncoming headlights grew large haloes and those classic smear-induced flares that blot out half the screen.
With Cornwall over 350 rain and spray laden miles away, and over a hundred miles of that at night. I was worried about the trip back home. Thankfully, it stopped raining just south of Lancaster and so my worst fears weren't realized.
I'm used to keeping my windscreen clinically clean and ensuring that my wipers are in good order. This gives crystal clear vision. If the screen picks up some muck that a wash/wipe won't clear, I only have to use a little elbow grease and screen-wash detergent - a process that takes less than five minutes and can be done on the hard shoulder if absolutely necessary. Now, if Rain-X treatment breaks down you need more Rain-X. I understand that properly applying this stuff (assuming that you carry some with you at all times) is painstaking. You must apply and then buff it correctly, otherwise you have permanent smears, and (crucially) I understand you can't apply the stuff in the rain!
So, even if Rain-X does what it says on the tin, it can (and as I found out, does) break down at the most inopportune times, when it become a dangerous liability.
With that said, if I can't get the wretched stuff off my screen, I'll be forced to use Rain-X myself (either that or replace the windscreen).