Thanks. It appears from that thread that an IOSS number identifies the registered marketplace, and thus might be equivalent to writing "IOSS eBay" on the parcel which is what I did.
I still don't get why the post office don't tell their staff what their own organisation requires.
This should help https://community.ebay.co.uk/t5/Seller-Central/IOSS-Numbers/td-p/7009589