More expensive wrong item delivered

AuraLight

I bought a desk and a desk chair was delivered. 
I emailed the company straight away informing them and asking for my item but after saying they would organise a swap, I have heard nothing. 
I have now emailed them saying I'm happy to keep the item if they can't organise the swap but the chair is more expensive than what I ordered. 

If they don't get back to me/take more than 2 weeks to organise the swap can I just keep the item and do they have the right to charge me extra?
In the box it's just taking up room in my flat that isn't really there. 

Aylesbury_Duck

You can't just keep it without their express permission to do so.  You can write to them asking if they'll hand it over to you formally, or that if they don't collect within two weeks, you will sell it and make the proceeds available to them.  They have up to six years to pursue the matter.  I suspect the ultimatum will spur them into action.