I tried the method that Alfred suggested and the resulting png was what I wanted, straightaway without any adjustments being needed.
Thank you Alfred.
The line is 15 point, which added the 10 pixels all around that I wanted.
That seemed strange as I know that 1 point is usually 1 and one third pixels in the Windows system.
So, on a hunch I tried changing the colour of the line and producing a graphic. As well as going outward making the picture bigger, the colour also went inward under the lettering, so all is well, 1 point is actually 1 and one third pixels in the image: it was just that in the first image made this way that the border going inward was not noticeable!
For completeness I tried the original method using the Arial font.
There was a white pale at the right, though not so wide as previously, and lines of a slightly different colour at or near some edges.
I hope to post a link to this thread in the Serif forum.
The Sonnet to a Renaissance Lady font is available as a free download from a link in the following thread.
Please enjoy using it. I produced the font myself so I am able to say that.
William Overington
23 May 2014