If you're that worried, remove the thermostat, bolt the housing back up, fill the system with water, drop some of http://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_categoryId_255217_productId_211647_langId_-1?cm_mmc=Shopping-_-Google%20Product%20Search-_-Car%20Care%20%26%20Maintenance%2FCar%20Parts%20%26%20Spares%2FHead%20Gasket%20%26%20Exhaust%20Repairs-_-Bars%20Flush%20Cooling%20System%20Cleaner&source=shopping in. Give it a good run, bearing in mind you're not running a thermostat so it'll take a while to warm up, not to mention run slightly richer. Once you've done that, drop the coolant again, flush with water, then fill up with the correct coolant, refit a new thermostat. Should give you some peace of mind. Day