I bought a SilentNight Miracoil Mattress from Argos 2-3 years ago for €399 (half price in a sale) and while it was really wonderful and comfortable for the first year, it is now fit for the skip, as the springs on one half (not the side anyone regularly sleeps on!) have collapsed. Anyone sleeping on that side can wind up clinging to the edge to avoid falling on the floor. I can't find the receipt or guarantee but definitely wouldn't buy the same again!
If you're going to buy this, be more careful than me and keep the guarantee and receipt with other important house papers!
Sleep tight,
Ms_Frugal