I have a 17.5 inch saddle on a 15h horse/pony and a large was way too big for us. I went for a medium. I was recommended the large for our saddle size but it was too big.
You should be fine with a large on a bigger horse
ETS: If it's any help I live less than a 5 minute walk from that shop and was popping in tomorrow if you want anything measured!
Thanks that's really nice of you, they are such a good internet site orders often arrive the next day! I think the large should be OK as Jack's saddle is 18 inches.
My saddler told me that the size you need depends on the make of your saddle eg I have a prestige and its a different size/ fit to my stubben. Could you take your saddle with you to see which is the best fit?