Basically, you can remove old one and replace with new,no problem,
If you leave the property for many weekends, holidays etc it might be worth you investing in one with an automatic mode, if you are not away a lot,it isn't worth the extra cost.
Economy 7 is the same thing, if you want a simple to use heater, look at "Creda" or "sunhouse" or even "Dimplex"
they are quite easy to install, if the old one is very old be aware that when you take it out it has fibreglass inside,so wear gloves, when putting new one in,make sure you get really good fixings to wall, you must follow instructions very carefully when assembling heater,and use a heat resisting flex to connect, I obviously can't tell you what size to install without viewing, but if the old one is over about 1 metre wide that will be equivalent to a 3.4 kw heater, good luck, Ray
if there is any more info you want please say.