could i have an air lock or could it be temp sender unit

i changed from a v1 to a v2 engine in my classic,new engine came with a thermostat so used it.....bled the cooling system but it seemed to take ages for fans to kick in & when they did it was for a very short period & heaters always blowing hot....so thought thermostat was shot so i changed it for my old 1 withc i know works now the fans kick in sooner but don't go off & heaters blowing cold!!! & seem to boil over but temp gauge stayed at half way where it normaly does,bottom hose cold so thermostat not opening....

checked old thermostat & it works ok so going to put it back in tommoro & see

where is the temp sender unit on a classic