"Does anyone know why the first stage used kerosene rather than liquid hydrogen & oxygen in the way that shuttle does given that the latter has a higher specific impulse so less fuel would have been required? Presumably the rocket would just have been too large or were there other reasons?"
The Saturn V was really pushing the state of the art - and the F1 first stage engines were several times more powerful than anything previously attempted (1.5+ million pounds of thrust). While H2 engines have a huge specific impulse advantage, kerosene is much easier to deal with since it's not cryogenic and H2 is far less dense that kerosene of the same impulse, so the first stage would have gotten even bigger.
The J-2 engines (using cryogenic H2/O2) were a huge advance over anything that had been done before using H2 as a fuel; even so, they only generated 110,000 lbs of thrust vs 1.5 million for the F-1. The SSMEs were, in turn, a huge advance over the J-2s, and even they only produced 400K lbs or so. If Apollo had needed H2/O2 engines for the first stage, it likely never would have made it to the moon.
If I understand matters correctly, it's also true that the specific thrust advantage of H2/O2 engines mattered less at lower speeds and denser atmospheres.