This tech tip is bestowed upon us from the MG factory. Referring to Confidential Service Memorandum MG/358 dated 24 March 1962.

If the surface of the transparent flexible sheet is not too badly damaged, the surface hair scratches may be removed in the following manner:

Using a soft sponge, wash with soap and water, rinse with clean water, and dry.
A smear of hard wax polish should then be applied with a soft moistened cloth, to give a veneer of wax and fill the scratches.
Finally polish with a soft duster.

Addendum: On 8/25/20130, Keith Arndt in Lyle, WA USA wrote:
"I'd like to add my favorite solution to cleaning and actually polishing the clear plexiglass". My experience is from small plane and motorcycle plexiglass. I've always used Meguire's #10 Cleaner followed by Meguire's #17 Polish. The cleaner doesn't work worth a damn on Lexan, but it works great on the softer plexiglass. My Robbins top has been on the shelf for 35 years and came out crystal clear after only a few minutes of work with a soft cloth. The only thing that remains are deeper scratches. I will try the wax trick to see how many of those disappear".