Bad fibre service

Absolute pathetic service in our area. They boast to double your speed on fibre during lockdown, that is so far from the truth it is actually unbelievable. I have a 10Mbps un-capped fibre line, the best I get on a good day is 6Mbps download, this on a supposedly upgraded line that should perform at 20Mbps! Today I have performance of 0,3Mbps, not on!