I'd replace the 'stat before going near the HCV if i were you. Before you start the drain, flush etc, ensure the heater is set to the hottest setting, it needs to be there so the matrix gets flushed too.
When you come to do it, invest in something like Comma X-stream Flush for the cooling system and add it as per the directions on it.
If it says run it at a fast idle for 20 minutes, take it for a half hour drive instead.
Once you're home and it has cooled somewhat, remove the 'stat housing and the old 'stat. Refit the housing then remove the top hose from the radiator stub. Use your garden hose in the open end of the top hose, if you have a Hozelock type fitting it should fit snugly inside the top hose.
Turn the water on for the hose and leave it running. This reverse flushes the system with the old stuff exiting via the radiator stub at the top. Usually takes about half hour to get it running clear, once it is running clear leave it 10 minutes or so more to ensure it really is clear.
Turn the hose off and remove it from the top hose. Remove the bottom hose from the bottom of the rad to drain off excess water from the rad and block and then refit - having a new Jublee clip for it is a wise move as they often rust.
Once the bottom hose is secure, add 5L of concentrated Ethylene Glycol antifreeze. The exact figure for 50/50 mix is something like 4.8L, can't recall it exactly and it's as close to 5L as makes very little difference.
Fit the new 'stat, note the seal sits around the edge of the 'stat ont on it or under it - there is a groove inside the seal to let it sit around the 'stat. Ensure the recess in the head and 'stat housing are clean and smooth, use some silicone grease in the groove and smear some on the outer parts of the seal too and fit, ensuring the jiggle valve is at the top if it has one.
Refit the 'stat housing and top hose to the rad.
Top up the expansion tank with fresh water (remember you already have enough antifreeze in so water is fine), refit the cap and squeeze the bottom hose several times to help move the air to the top. If the level drops in the tank, add more water to the "MIN" line and squeeze the bottom hose a few more times. Repeat until squeezing the bottom hose doesn't result in further drop of the level.
Refit the cap and take for a drive, ensuring you get it up to temperature and that the heater works, no leaks etc. Return home and park it facing uphill and leave to cool until the following day then check and top up the level to the "MIN" mark if needed.
I've used this method for decades on a variety of cars and it works. Whatever you do, don't be tempted to buy OAT coolant, doesn't matter how good the salesman says it is, it will destroy your engine.
If some idiot has previously used OAT coolant (usually red or orange) then you have other problems and could be the cause of your lack of heat. Hopefully they haven't though!
