Refund for Item sent in error by seller

[jaydee303]

I ordered a blue mobile phone from a company in China.

They messaged me that they only had it in white and asked if I would accept a white phone.

"If it is fine with you,please let me know and I will change it for you.
Anyway,if you don't like it,please kindly let us know,will issue you the refund asap."

I replied

"Sorry to hear that, but I don't want white, so please cancel the order and refund
May Thanks"

I then ordered the phone from another listing which had a blue phone.

They then found they had got stock of blue phones and sent one.

I ended up receiving two phones.

They admitted their mistake and appologised.

However, they will not refund my money, despite their promise above.

They will not not collect their unwanted phone.

All they have done is send a PDF file with a return label.

They refuse to make a refund without a tracking number.

They refuse to collect the unopened package, which is their property.

I cannot afford to post it to their fulfilment centre here in Uk, due to my work.

I also do not wish to do their job for them.

They seem to have a bad track record of resolving problems.

I have an unwanted phone and they have kept my money.

I cannot find how to open a case where a seller mistake has led to 

a situation like this.

Any ideas on how to do this ??

Thanks