Again to my own question - Just found where to go in Seller Hub - Orders>Postage labels>Actions (dropdown)>Manage delivery with Sendle.

Just replying to my own post - I never found where to cancel from the seller hub, but I did manage to cancel. I called Couriers Please who are on the Sendle postage label (Sendle must run through them). They sent me a link for an online chat and the agent who responded to the request cancelled it (by sending a message to the driver not to collect) and gave me a reference number to use if needed. To keep in mind though the link they sent for the chat has to be used straight away (only valid for 1 minute because it is a single use chat link).