Caroline, I don't know why either...the thread took a detour.
Anyway, to go back to the OP's initial request, for info on getting TO Heathrow, I did see the van taxi guy in Maidenhead today. Normally, he charges £25 Maidenhead-Heathrow (coming or going); for a pickup in Windsor instead, it's £30, not bad for door to door service. The van can definitely handle 4 adults and their luggage (assuming you aren't all supersized). Call Parvez at Prime Line Taxis; his number is 44 (0) 7884 120 342--to call him within the UK, dial 07784 120 342. You don't have to book far in advance, a day or two ahead at most should be plenty of notice. (If there weren't 4 of you needing a larger vehicle, a regular taxi wouldn't need to be booked more than an hour in advance.)
If you find van taxis in Windsor, you can use this price quote as a comparison.