Whilst a light aircraft was lined up for departure, a vehicle made an incorrect assumption about the nature of an ambiguously-phrased ATC TWR instruction and proceeded to enter the same runway. There was no actual risk of conflict since, although LVP were still in force after earlier fog, the TWR controller was able to see the vehicle incursion and therefore withhold the imminent take off clearance. The subsequent Investigation noted that it was imperative that clearance read backs about which there is doubt are not made speculatively in the expectation that they will elicit confirmation or correction.