With a bit of fiddling and help above, here is the answer:
http://jsfiddle.net/hslincoln/8e5Vr/36/
jQuery(document).ready(function () {
var handyTotal = jQuery(".price").text().replace(/\u00A3/g, '');
var aPrice = handyTotal / 3; // takes basket total & /3
var aPrice2dec = parseFloat(aPrice.toFixed(3)); // converts to number with 3 decimal places
var bPrice = handyTotal / 3;
var bPrice2dec = parseFloat(bPrice.toFixed(3));
var cPrice = handyTotal / 3;
var cPrice2dec = parseFloat(cPrice.toFixed(3));
var bPriceFloored = Math.floor(bPrice2dec); // rounds down number
var cPriceFloored = Math.floor(cPrice2dec);
var overflowbPrice = bPrice2dec - bPriceFloored; // finds amount to add to a
var overflowcPrice = cPrice2dec - cPriceFloored;
var calcaPrice = jQuery(".Instal1").text((parseFloat(aPrice2dec) + parseFloat(overflowbPrice) + parseFloat(overflowcPrice)).toFixed(2)); // adds b+c to a
var bPriceFlooredDiv = jQuery(".Instal2").text(Math.floor(bPrice2dec));
var cPriceFlooredDiv = jQuery(".Instal3").text(Math.floor(cPrice2dec));
Thanks all!