Octaviamk3

Evening guys, Got an issue with my 2016 Octavia, 184 Diesel. 131k. I've been loosing coolant recently, around 0.5L every couple of hundred miles. Changed the cap, no signs of leaks. I put some paper under the overflow/overpressure relief of the expansion tank and it did have a wet spot after a drive out. As it seems the tank is over pressuring and relieving, all I can think of is the head gasket has gone. The coolant is still a redish colour, no mayo under the oil cap. With the cap off the expansion tank, the level doesn't rise and fall when the engine is running, or bubble. Before I go to a garage, is there anything else it could be, a search doesn't really bring up any answers online. Thanks in advance.