Thanks for your help.
When I run the VBA macro in Excel it identifies the copied graph (a histogram) on the clipboard as a picture (i.e it only executes the If fmt = xlClipboardFormatPICT line)
On the Mac version it will Paste into Designer as a fully editable vector drawing, each bar can be resized, moved, change pattern etc.
In contrast, doing exactly the same process on the PC version it always ends up as an image.
All of the Paste Special options also end up as an image.
However, if I then paste into PowerPoint without clearing the clipboard it pastes as an editable Microsoft Excel Graph Object, not as an image so the vector information I am assuming is still there. It appears like Designer running in the Windows environment ignores the format of the object on the clipboard and if it is not text then assumes it is a bitmap.
I think I will have to get my students to email me their spreadsheets (usually very large) so I can copy and paste the histograms into AD files on my Mac and then email the files back to them. They are able to open the Mac generated AD files on their PC version and edit them as normal.