msd do a uprated fan switch for 20 quid or so which makes the fan come on a little earlier and go off a little later. But for your switches to be melting means that your fans are ddrawing too much current. Then only drawback with the msd fan switch is that your fans are on more often and if you have a problem with your fans or wiring will just amplify it somewhat which could lead to burning out again. Has your rad got plenty of clean air as you say you got twin fans means you got a full front mount is this totally obstructing the rad