the question isn't whether the sample is big but whether it's representative. YouGov are serious pollsters who do these things properly. It can't be perfecly accurate but you'll usually find a margin of error built in.
YMB:"That Scotland will stay and we will have even more years of whining. " - The problem is that it seems that the majority of Scots do not want to leave the UK but the minority that do whine the loudest so yes no doubt we have many more years of whining to come. The sad thing is that they don't actually want "independence" anyway they just want to be independently subjugated by the EUSSR.