Assuming you are going on Eurostar, you can buy a ticket that will originate in Leeds right through to Paris on the Eurostar site, but before you do that check on the price from Leeds to London seperately. If you are travelling on Eurostar (and have your ticket or e mail confirmation) you can go from Leeds to "London International CIV". Using this destination means you are going to use Eurostar and the Leeds - London bit of the journey may be a bit cheaper, but the best bit is there are no time restrictions, so you can travel at off peak rate during peak periods (which may help you if you have to leave home early to get down for a, say, 1030 train to Paris). Only downside is that you cannot order this ticket on the internet as you have to show your Eurostar ticket to the ticket office as proof. More info at
http://www.seat61.com/London-Paris-train.htm
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