I would try and fix the leak rather than use stop leak. However if you must do, check compatabilty with dexron 3.
Here is halfords
http://www.halfords.com/webapp/wcs/stores/servlet/product_storeId_10001_catalogId_10151_productId_175466_categoryId_165750_langId_-1?cm_vc=IOV4PLPZ1and if you have a eurocarparts near by it is half the price
http://www.eurocarparts.com/ecp/p/Accessories/Lubricants-and-Fluids/Tranmission-Oils/Transmission-Oil/?522770572&0&cc5_309If you still have the bottle you used you could check the label and see if it was dexron 3 or not.