In my case, the PDF is just for myself so I know which bills I have paid and I'd rather keep it as a PDF with original text elements that can be searched etc. As I can rotate the "stamp" using the trackpad on my MacBook trackpad I can achieve what I want in the end. I was really just curious if there was any way of doing this without a trackpad when I am working on my iMac.
As others have pointed out elsewhere it is possible to rotate text to 90º vertical in Preview by first rotating the whole page in Preview, then adding the text (which will be horizontal) and then rotating the whole page back to its original orientation. The text will now be vertical. But there is no way to do this (without a trackpad) if you say wanted the text at 45º.
It would be nice if the next version of Preview would allow text to be rotated using the mouse or by typing in an amount (but I can't see this being very high up on the list of things for Apple to do).