P38 Radiator Problem

Wonder if anyone can help with this problem. About 12 months ago I was towing a 2 tonne caravan up a steep forest track, everything seemed fine until steam started to appear from under the bonnet, when I looked I noticed a hairline crack in the radiator top tank (plastic). I replaced the rad and fitted a kenlowe fan at the same time.
Everything was fine for about 10 months when I started to lose coolant and noticed the radiator bottom tank had come away from the matrix. The rad was replaced under guarantee so I fitted the new one and a new cap at the same time. All has been well for the last 3 months but coming home tonight I noticed steam from under the bonnet and found that the top tank had split along the ridge. Any ideas? Would have thought a dodgy head gasket would have shown up a lot sooner. Thanks.

At a guess too much pressure.

Hi Beast of Bodmin, I've got a Laserline pressure tester but can't use it now as the rad has a spilt in it. Any idea why the radiator cap wouldn't release the excessive pressure. Don't want to fork out for a new rad and spilt that one too. Cheers.