Its a difficult question Naomi, and no mistake !
An important factor in the sudden rise in popularity of the SNP may be reflected in the distaste for the Tories north of the Border. All three main Parties were active in opposing the YES vote, but it was a Tory PM they are blaming. Revenge upon the Tory Party must be taken into consideration.
By the way, the SNP will be capable of "holding sway" over the Tories, just as easily as Labour. As it stands today, out of the 59 MPs that Scotland sends to Westminster, 58 of them are Not Tory.
The Tory Party has had many years to recover from its wipeout in 1997, but an increase of just one seat doesn't seem to show much in the way of progress to me.