'Service Vehicle Soon' message and Engine Fan

Hi, I have the vehicle listed above it had a major service about 4-5 months ago. Recently, the 'Service Vehicle Soon' dashboard message comes on and off seemingly randomly while driving, and sometimes it comes on as soon as I turn the car on. Every time the message appears, the cooling fan in the engine turns on (and then when the fan turns off, the message disappears).

I did take the car to one independent garage, and they reported "Fault code read p0597 engine coolant thermostat heater control circuit. Have tried to communicate with system on actuator side not working. Cooling fan running when ignition is even on mar position requires wiring checks." They wanted to charge me an additional £130 to do a couple more hours of investigation, and I decided to stop there, thinking that probably I just need to take it to the Vauxhall dealer for something involving codes. I can't get an appointment with the dealer for a week or two.

From some internet research, it seems like my problem might be the one identified by TSB 2610: “SVS illuminates, P0597 stored in engine control module. Sometimes cooling fan runs permanently. Cause: Poor contact of the heating element of the thermostat causes the trouble code. Remedy: Replace thermostat housing."

I would greatly appreciate your advice, particularly in terms of:

1. How likely is it that my problem is the one identified by TSB 2610?
2. How urgent is it that I get this fixed? Is it harmful that the fan is running a lot of the time while driving? I typically drive about 20-25 minutes per day.
3. Is replacing the thermostat housing something that a non-professional without specialized tools could do? Or would that be dumb?
4. Am I right to think that it’s best to take it to the Vauxhall dealer because they have the specialized proprietary equipment for connecting to the car and diagnosing this sort of issue?

Thanks in advance!