That diagram is not the same as my outboard I don't think. On mine, the water is pumped up to the thermostat where, it can either flow down the side of the block to the tell tail exit (when the thermostat is closed) or flow down there and down the end of the block around the plugs and then I assume out of the outlets at the bottom of the leg (when the thermostat is open)...hence why if I run it without a thermostat, excess water flows down the end of the block and out the bottom leaving only a small ish amount to flow down the side and out of the tell tail. This is all an assumption as I've only been able to look at the engine from the outside.
New thermostat is £32.50+VAT+carriage
I'm going to double check mine with a thermometer before ordering. I'm not sure they do ones for different temps.