Bottom Entry float valve compatibility?

I'm having issues with my bottom entry float valve taking an eternity to refil the cistern. Went down to B&Q where the toilet was purchased from, picked up a new valve and went to install this. The thread was too wide for the hole in the cistern. On further inspection, i noticed that there is a nut that screws on to the valve thread and then the flexi flexi hose then screws on to this. Does that mean that my valve is a 3/8" one?