If you are not sure turn the small one off (across the pipe) under the pressure reducing valve.
but it should just be the the one labelled "cold water mains isolation to hot water cylinder".
Open the hot taps (lowest one such as bath) wait until the water stops running then twist the T&P knob, if your cleaver you might get it balance in the open position before it clicks over on the cam, if not you will need to hold it. This will allow the air in & the water out of the tap . You may need to keep this going for 10-15 mins until the water stops running & the air gluging-in.
Turn T&P to closed, shut hot taps, open up the isolation valve refill the system, open hot taps to vent the air in the pipes.
Most important dry out the tun-dish & check that water has stopped dripping & again in a day of 2, if it is still wet after two days then you will need to call an Un-vented plumber to check over the system.