I checked around with the IR meter and when the boiler claims its supply is 55°C, then the pipes (many types, sizes, etc.) in the basement all above 51°C, while the ds18b20 was still registering 42-44°C
you could use a heat conductive glue/pasta that is also used for processors and the cooling aluminium thingies.
Or if you use tape, you can take a iron file [metalowe opi?ki ?] to make the DS18 a bit round matching the diameter of the copper pipe. (maximizing contact)