No, you can do it all from above. The only picture of mine I can find with the part in is one I did for the tps sensor. Ignore the big red arrow (that is the tps sensor). The flow control valve is circled in green, and you can see the operating cable in the pic.
Check the cable is still clipped on. As you adjust the temp inside the car, you can see the cable operating the valve. Just because it is moving the valve, doesn't neccesarily mean it is set correctly though. You can unclip the cable and move the valve manually to check the operation. i.e. disconnect the cable, put the interior fan on (once the car cooling system has warmed up of course) and operate the valve manually over different positions and check whether you get heat or not. If you establish you can get heat, put the interior temp up to max (so the cable moves to the max position) and then clip the cable back on with the valve in the max heat (from previous test) position. It should then work correctly again when you tirn the temp up and down.
If need be, you can flush the matrix by either undoing the pipes at that valve, or by following them to the bulkhead and disconnecting there. Like I mentioned, do be carefull not to p*ss water all over the top of the engine, as water down the plug wells is a pain to get to and dry out (don't ask me how I know ).