Hi,
I took out a new mobile using a similar company 2 years ago and it was nothing but headache. Unfortunatly, many of these companies go out of business, do not even contact their customers and run with your money. My advice to you is to find out if the company are actually still trading by trying to call them or email. I would also check with your network if they still are in contact / business with the company and if not when they parted ways.
The networks do not accept any responsibility for these partnerships and will not refund you your money on their behalf. To be honest, if they have parted ways, unless your insurance covers you to get out of your contract early, the network will still expect you to cover the cost of your remaining months line rental, irrespective of the cash back deal with the third party.
Having said this, my personal experience was about 2 years ago and so it is possible there there are new methods on dealing with this. Try other money saving website forums and see if there is any further information that might help. I think Money Saving Expert wrote an article on this not too long ago.
Hope this helps