I see that it "feels" excessive. However, read the Judge's sentencing remarks here
https://www.judiciary.gov.uk/judgments/sentencing-remarks-of-hhj-dutton-r-v-gayle-newland/
It seems aggravating features were planning, breach of trust, deception, crime committed in victims own home, the unusual nature leading to ridicule etc. The problem will be that it is going to be difficult to set the sentence in this case since there is no polarity with other cases.
How was the victim taken in? Quite simply, she was lonely and vulnerable and wanted to be loved. Some lonely and vulnerable people are ripe for believing anything their abuser tells them.