Bubbling coolant

I posted regarding this months ago though the fault seemed to sort itself,so I thought. Yesterday when I parked I popped the bonnet to find a bubbling sound coming from the expansion tank. Last night I had to top up the tank as it was empty. The coolant colour is still nice and blue.

I am going to change the thermostat as I cannot find any leaks in the system.
Could a techy please instruct me as how to go about doing this and advise me on anything else I should check. Would a pressure test be worthwhile ?

Thanks in advance :thumbs up: