Dear Sir
I have created an Arabic font with FC and it is working perfectly.
The problem comes when pasting a mixed text like English and arabic, the Englesh letters disappear until I change the font of the text.
Yes the font I have created does not contain any English characters.
My question is, Are you aware of any way to get the font to substitutes the English characters with a default font? 
Thank you very much in advance for your reply.
My question is, Are you aware of any way to get the font to substitutes the English characters with a default font?
No. However, my knowledge is not that great so maybe someone else knows otherwise.
However, I have seen an effect that might be what you want which is that I was using a Microsoft application package named Picture It! once and that application did substitute a character from another font for a Private Use Area character which was problematic within my own font at that time. However, I regarded that as a feature of the application, not a feature of my font or the other font. So, as far as I know, that end result would be a feature of the application package using a font, finding that that font did not have a glyph for a particular code point and the application package then obtaining a glyph from another font.
Does anyone know anything about packages which act in this way and how they do it please?
William Overington