You won't get too far looking in the IGI for Devon Parish data because a lot of the parish records are not transcribed there, for example Awliscombe, in which Viv is interested, is NOT on the IGI.
And even if your family do live in a parish which is on the IGI, the fact that many parishes were within a mile or two (or even less) of each other means that some baptisms and marriages are bound to have been celebrated in a neighbouring parish which isn't on the IGI. Poor people also moved around a lot and, inevitably, came and went between the different parishes.
This notion that the IGI must contain all the Births and Marriages has resulted in a lot of mistaken identities.
However, in Devon, we are compensated to some extent by having an excellent Genuki site and Family History Society. The Devon Record Office and Plymouth and West Devon Record Office have also been very good at putting lots of data onto the National Archives site which you can search fairly easily and then write off for photocopies.