I think IKEA were having a sale recently with very cheap mattresses, maybe check there?
Having said that, I really think you should stretch for a really good mattress, I'm normally always looking for bargains and was going to buy a cheap mattress but my brother persuaded me not to and I got some really good ones in kingsbury furniture in Tallaght, will check tonight on the brand - they were Irish made which was also a plus.
I notice that the mattress you linked to has memory foam, mates of mine spent a fortune on a full memory foam mattress a few years ago but found it very warm - I see this one states the coils dissipate the heat, so maybe they've improved.