[mothered]

All got delivered in the same box by DHL. Ordered 2 drones, a bag and a gimball and I said only 1 drone came

What's the weight of the Item you claimed as not received?

They’ve investigated and refused a refund even after I provided a police report due to there being sufficient evidence of delivery.

What exactly Is "sufficient evidence?".

and would normally take it further to the head office but I cannot find any contact info online

Simply ask them for the contact details, and/or request your claim be escalated to their senior department.

Weight was 10kg but I heard DJI was pretty easy to do.

Sufficient evidence that the whole parcel was delivered with the correct weight.

I have done this and they have said the investigation is closed.

It's of no surprise your claim was declined.

You cannot possibly use the Item missing method on something that weighs 10Kg.