Don't be put off by their response. The mattress you purchased comes with a five year guarantee. They do try to limit thir liabilities by a mention in the guarantee that if one uss the mattress on an existing base, the guarantee may be void if the base itself is damaged or worn at the outset.
If the base was fine and firm (not sunken in), then under your statutory rights you have a valid claim.
I had one of these which sagged after a few weeks. I put up with it for just over a year then reported it as an inferior product. They arraged for it to be inspected by their appointed "FIRA technician".
If they asked you to place a pole (broom handle) across the middle of the mattress and measure the gaps underneath it, then this is an incorrect method of determining the amount of sagging. I had a good laugh when they asked me to do this, which was also addopted by the FIRA technician. But the FIRA guy used a plumb-line instead.
If one places a plumb-line across the middle of the mattress and you see a gap underneath it, it is not a true measurement of the sagging. The reason for this is very simple, if one is to place a plumb-line across the centre and then another along the mattress approximately central to when one sleeps, one will see a significant gap between the two plumb-lines. Therefore, the true measurement of sagging is taken from the plumb-line placed along the mattress to the lowest point in it.
One sleeps along a mattress, not across it.
A mattress should only show normal signs of wear. After a couple of years one would expect this to be one to two centimetres at the very most and it's supposes to give you the support you would expect during your sleep. The whole purpose of using memory foam is so that when one changes position it should recover within a few minutes, about ten to fifteen, to support the new sleep position.
From information gleened from the forums about this product I think there is a common fault with the Relyon Airflow 5000. If I remember correctly, it was supposed to have pocket springs and also two layers of memory foam. Sadly, the manufacturer has got it wrong this time.