2010 Vauxhall Insignia overheating engine

Recently towing caravan and the engine overheated after a half mile incline.
All the warning messages for Water / Oil Temperature came on. I spotted temperature rising towards the top of incline and pulled over as soon as possible. There was evidence of water around expansion tank. It was very hot. It took about 2 lires of water to refill it slowly and this nearly boiled as it started to go in. After it had cooled down, the engine was restarted and eventually the temperature guage cooled down and it took another litre to top up. I continued the journey, another 100 miles in 30 degree heat and back home 120 miles 2 dayd later - another 500 miles since and temp guage is normal. What could make it boil over? I just don't trust it for towing now. It may be my imagination but it seems to be a bit down on power since and mpg has dropped a couple of points and only getting 26 mpg towing. Car has only done 36k. Any ideas ?