Radtec radiator is about £200 (L7OC bulk buy to get that price). Alternatives are the Caterham ali road rad or their triple pass racing rad. Search on here for comments on these. I went the Radtec route because it seems to be more efficient than the Caterham road one and cheaper than the racing triple pass job. Some people have concerns over the flow rates achieved in the triple pass rad as the cores are smaller.
Fitting is a doddle - disconnect fan, disconnect lower radiator hose making sure you have a suitable reeceptacle for the coolant, disconnect upper radiator hoses, unbolt radiator (4 bolts), ubolt fan from radiator (4 bolts) then reverse the process and refill with coolant. Bleeding can be a pain but get the front of the car as high as possible to remove airlocks (search techtalk for more info on this).
Yellow SL #32