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.
Back again with another post office question:
I sold something through eBay. The buyer, located in Europe, paid VAT to eBay as part of the transaction.
The post office website says, for this situation (IOSS through registered marketplace):
I tried two branches and neither knew what this meant.
So my question is: what is an IOSS number, and how should the post office be allocating it?