The only sentence that can be passed for murder is life imprisonment. The Judge did have to determine the ‘tariff’, the period of time that Ms Smith had to serve before the Parole Board could consider her case and see if she could safely be released (see here for an overview of how tariffs are calculated).
We do have the sentencing remarks of the Judge (Griffith Williams J) who had presided over the trial. Whilst it was the jury who decided (in this case on a 10-2 majority), they play no part in the sentence. It is worth reading the remarks to understand why the tariff was set as it was.
Briefly, however, the starting point for more than one murder is 30 years. The Judge would then have to balance the aggravating and mitigating features. Here, the fact that there were five, rather than two, deaths is the main aggravating feature. The Judge decided that this cancelled out the mitigating factors – the fact that Ms Smith had not been in trouble before and that this was impulsive.
In all the circumstances, this seems to have been a just approach and there is unlikely to be any appeal. As was mentioned in the comments on the previous story, this is not so obviously a case of murder (as opposed to manslaughter). It is likely that there will be an appeal against conviction and this should become clearer if any when that is heard.