It's not clear to me if the problem radiator is getting hot or not. If the radiator is getting hot (and 70C means about too hot to touch) without warming up the room properly then, as already suggested, the solution is a bigger radiator. If the radiator isn't getting very hot then the problem is a constricted flow. Check the pipes going in and out. One (the supply) should be much hotter than the other (return flow).
First check if the radiators is properly bled and there's no air impeding the circulation of hot water (radiators which are warm at the bottom and cold at the top are a symptom). However, in my experience, it's usually an upstairs radiator which collects any air.
Then unscrew the thermostat off the thermostatic valve (if there is one) on the problem radiator. Sometimes the valves get sticky and the pin in the middle needs a bit of brute force (eg pulling up with pliars) to loosen it. Check that this pin moves up and down by a few mm freely (you need something solid to push it down but a spring should then push it up again).
The next step is to open the lockshield valve at the other end of the radiator by a couple of turns and see if that helps the water flow. More flow = hotter radiator. If you still aren't getting enough hot water coming into the radiator then open the lockshield valve some more. If the radiator isn't very hot with the valves fully open then most likely the pipes need a power flush. Not all homeowners (or plumbers) are diligent about keeping the system well dosed with inhibitor (eg Fernox).
I basically agree with all that's already been said about the need for big enough radiators with condensing systems as they are most efficient when running relatively cold as they can then extract a little more heat from the furnace. I've got an outside temperature sensor on my system which automatically changes the target boiler temperature according to the expected heating requirement based on the outside temperature (when the outside temperature is below zero the boiler temperature is about 60C but if it's 10C outside then the boiler temperature drops to about 45C. A consequence of this is that the heating water gets pretty tepid by the time it reaches the far end of the house and the problem is exacerbted by the burner cutting out once the circulating water in the boiler exceeding the target temperature. The burner won't restart until the circulating water was dropped by 10C which can take a while, particularly at the lower temperature end of the range.)
I suggest you get an infra-red thermometer eg
https://www.ebay.co.uk/itm/Digital-Infrared-Handheld-Temperature-Gun-Thermometer-Non-Contact-IR-Laser-Point/251885627064 so you can quickly check the pipe (and therefore water) temperatures at various points in the system. When everything is heating up with a properly balanced system the temperature difference between inlet and outlet of each radiator should be the same (around 10C). The thermometer is also very useful for finding any cold spots in the house (eg poor insulation or drafty windows). Just point it at something and it will give the temperature.