Unfortunately you have experienced a very common problem with your make of mattress.
Over recent years many of the big manufacturers have lowered their quality in order to cut production costs. And some manufacturers have just gone too far, making many of their products unreliable. In fact the Miracoil spring system is their own name for a continuous spring system. Many manufacturers used to use this type of spring system, but most have stopped due to them being very unreliable and noisey.
If I were you I would stay well away from big brand names and big retailers if you want a good quality mattress at a low price. Instead go for a smaller independent manufacturer, and buy on line from a retailer who gives a proper satisfaction guarantee, that allows you to unpack and sleep on the mattress.
As far as memory foam goes, the mattresses that are all foam (memory foam on top of reflex foam), can be very hot. You are better off going for one that is sprung with a layer of memory foam on top (or better still a two sided one that will last far longer). Therte are also climate control fabrics available like Climasmart.
This site may help you. It is just an advice site, and isn't trying to sell you anything: www.bedlum-beds.co.uk