King Size Mattress review: Mattress

Hiya. We bought your kingsize hyllested mattress less then one year ago and we have reguly turned it around and is now saging on both sides. We are not heavy people, I im only 12 stone while my girlfriend is 9 stone and it now getting to the point where we cant sleep on it any more unless its in the middle of the bed when only one person is in the bed. Of cource its impossble to see this in any picture, you would have to lay in the bed to feel the differance. We cant carry on with this much longer as its not giving us the best of sleeps anymore as we did when we first got the mattress.
Can you please help.
The order number is
Darran Page
22 Conway Road
Paignton
TQ45LF
TEL [protected]
Tax Invoice for order [protected]
INVOICE NUMBER
GBINV21000000398372