Where was the steam under the bonnet coming from ? There's obviously a leak there somewhere, or it could have been just a blocked breather cap on the expansion bottle. Any residue of oil / mayo in the expansion bottle or dipstick ? My guess is that it was the headgasket even though you couldnt see why. The new rad will help circulation, and you can check the thermostat is working by removing it and boiling it up in a pan of water to see if it opens. good Luck