WARNING - read entire post!
I bought one of the £189 mattresses in May for £169 with £20 off - it is excellent - supporting, no sagging, good quality etc. Even has a zip off mattress cover.
However, based on this I bought another one for our spare bedroom because they had marked them down - £189 to £109.
Big mistake - identical packaging but it's a switch job - the 2nd mattress is poor quality, not 100% memory foam - it's a terrible cheap soft foam mattress with an inch or so thick memory foam topper built into it.
I've currently got a complaint in to Poundstretcher but no reply other than a 'we've passed this on to the relevant team'.
I'm sure they're unaware of this problem too as Poundstretcher, despite being cheap, seem like a reputable company - seems like it's possible their supplier has duped them.
I will post again if I get a happy resolution - ie refund or replacement with the genuine article.