The Contact page on the blog had stopped working and so I had to remove it and insert a new one. A simple enough task once one knows which bits of HTML to edit (there are plenty of blogs out there with useful instructions too).
Anyway, having got the Contact page back up and running, the "Submit" button was really bugging me because it was too wide, but worse, the text was partially obscured.
As the prosecutor in The Trial on The Wall by Pink Floyd said: "This will not do!"
It took an age to work it out (I couldn't find any help online), and it led to many unusual looking contact forms, but eventually I hit upon changing most of the values from 100% or 100px to auto, et voila. Perfection!