Hi Tamara, 
 
Thanks for your question and sorry to hear of your issues. 
 
The product, being purchased in September of 2017, is well outside of it's 60 day love it or return it guarantee. The reason we give that guarantee is for this very reason. It's a no quibble policy meaning that if the product isn't right then you have the option to send it back for a full refund or utilise the one time exchange. 
 
With Zip and Link mattresses we do always advise that you will notice that it is 2 mattresses and the only way to get around this is to add a mattress topper. 
 

 
Please note that with dips, natural settlement is a dip up to 1.5". Anything outside of this would be classed as unnatural. Please see the article here for more information on settlement. 
 
My best advice would be to try a different mattress topper. Maybe have a look at a company called Soak and Sleep - They retail a wider range of mattresses toppers than we do.
 
In regards to the rolling into the dip - If the mattress is on a sprung slatted base then that would explain the dip. If this is the case then try boarding the base with some MDF, plywood or pegboard to solidify the surface and prevent that roll into the middle. 

I hope this has answered your questions fully but if you do have any further queries please don't hesitate to contact our office on 0161 437 4419.
 
Kind Regards, Lee.