VRS tdi limited edition coolant/thermostat issues

Hi guys, i'm new to the forum!

I've been the owner of a Octavia 2 tdi 170 limited edition for the past 6 years and recently came into a few issues with coolant temperatures...

The problem i'm having is, the car will get upto 90°c when driving along, then when i start slowing down or if i come to a complete stop, the coolant temp drops pretty rapidly down to either cold or almost fully cold, then once i start driving again, it slowly climbs back up to 90°c... i've been having this issue for the past year, when the temp sensor popped out of its holder (due to the clip breaking whilst replacing the turbo oil feed) and it pished coolant everywhere... i bought a new sensor and housing and fitted it, ever since then i've had the issue of it not holding temperature! I've replaced the thermostat at the front of the engine and it didnt make a difference, and also replaced the coolant pump B, due to it throwing up a code (open feed fault) and i've also had vag com plugged in on live data, no codes and it told me the water temperature the same as the needle was showing (70 on vagcom and 70 on needle etc)

Euro car parts tell me theres 2 thermostats on my car? 1 at the front of the engine which i've replaced... and one in a housing with 4 pipes going it to...? I cant for the life find that second housing on my car... could any body please help me locate it? 

The cars currently in bits at my dads house, as i'm putting another new thermostat in incase the last one was faulty (both TPS parts, so they should be good...)

I have the 'second thermostat' waiting to go in... but i cant see the housing absolutely anywhere?! 

Its a manual 2.0l tdi 170... 

Could anybody also tell me the engine code for my car? As i'm absolutely clueless haha,

Thanks in advance, 

Rob!