Was the water comming from the thermostat housing when you looked under the bonnet..where the stat was changed? It could be that the housing wasnt properly sealed closed after it was changed and the coolant may have boiled away/evaporated....and then with little or no coolant left the engine overheated...etc
If the head gasket has gone there may be oil in your water and water in your oil (pale brown mayonase) usualy can be found in the filler cap. Also I think a garage can test your exhaust gasses and compression ratio in order to see if the head gasket had deteriorated.
Its hard to say what the extent of the dammage is ...if any.
A repair could cost into the hundreds depending on the dammage...this might be an option for you to save a few quid.
http://cgi.ebay.co.uk/NISSAN-MICRA-1...QQcmdZViewItem
If it seems ok you could top up on coolant and lubricant and see you it goes around the houses...risky though
