I had the same problem with my old s2 a garage told me my head gasket had gone, but some one told me to try taking the thermostat out, so i did and it was fine ever since.
I'd recomend doing this first all you need is a 13mm socket and it only takes about 5 mins to take off the thermostat housing and put it back on (be carefull though theres a paper gasket between them, but then a new one shouldnt cost much if you tear it)