Suffering a Hypnos pillowtop mattress and need a replacement

I have suffered a Hypnos Pillow Top Superior mattress for 18 months and am considering your Artisan Naturals soft tension as a replacement option. Marie has provided good advice and confirmed that this model would suit my 1750mm x 68kg bony frame.
Apart from the discomfort of the Hypnos, I have noticed pinkish staining below my pillow mostly in an irregular pattern but with a couple of distinctive circles one 80mm and one 40mm diameter.
One of your excellent posts touches on the risk of staining resulting from compaction and I was wondering if you can identify my symptoms with this cause.
Do you consider this type of staining as a defect which would justify a claim under the guarantee? As mentioned, the bed is only 18 months old and the stains were first noticed 3 months ago.
Your site is so refreshing compared to the hype filled over exaggerated publications of other bed companies.