To ask who's responsible for this

[parcelpuzzle]

I have bought an item from eBay and just got home to find a Royal Mail card saying parcel left by side gate.
However there is no parcel by the gate.
Is this something I need to take up with Royal Mail or the seller or the police?

Any advice please?

Royal Mail.

You tell the seller. It's up to them to get the parcel to you. They are the one with a contract with Royal Mail, not you.

With the seller who will take it up with Royal mail.

I would be polite about it but point out if fare isn't a secure place or hidden. Give them info they can use with royal mail

THE POLICE????? This is a joke post right?

We'll presumably I'd need a crime number as it's a possible theft.

Incorrect.
OP's contract is with the seller, sellers contract is with Royal Mail. OP needs to contact the seller.

The seller. When yiu purchased the item, you entered a contract with them. They have not met there part if the deal.
Seller will have to send another item or 100% refund you. Then seller makes claim with royal mail