Hi, I know I can upload a photo to a post I make, and have done on several occasions, but can I use the same method to add an Excel page in a similar method please?

I would guess, that a direct upload of Excel files is not ideal, because of the nasty VBS-Macro stuff that can potentially be put in these.

Why does it need to be an Excel sheet, though? Should other people be able to edit the file too?

One option could be to save your Excel sheet as PDF and upload this. Then people can see the content.
Second option is Google Docs that can be linked to directly from your post and you can allow other people to edit the content.

The only 'attachment' you can add to a post is a PDF so, as MacRat says, best to do that.
Nobody can mess around with your Excel sheet then.

Yep, post as PDF.