So further update:

After asking for my complaint to be escalated I received this reply this morning:Thank you for coming back to us. I appreciate you wish to escalate this matter further and I'm pleased to reply to you in this reference.

As previously mentioned, If our staff find any item, they’ll take it to the airport’s lost property office. You can contact the lost property office for Hamburg on[email protected].

Unfortunately, we’re not responsible for any items that were in your care while you were on board, and so can’t cover your claim this time. However, you may be able to claim from your travel insurance.

I’m sorry you had this experience on your trip. We hope to welcome you back on board soon.

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As their agents have confirmed they have the iPad in their possession I suppose the next step is to contact Hamburg police? Does anyone have a phone number for BA complaints before I take that next (rather drastic) step?