I discovered that when I first created the form, so all was well. Then I changed some settings, saved, published and re-inserted the form on a VSD page.
So the label box is still empty.
I tried deleting the button image and then re-inserting it, but the text is still there. It also has more text this time – it just had 'submit' over it the first time, now it looks like there's another word after submit.
The form (and website) is on SDrive – I hadn't planned on messing with any form code