I found a pattern book for Marriner's Heritage listed in a British auction site. It is for a close-fitting ladies' turtleneck in sizes 34 - 40 that calls for 8 - 10oz (presumably the difference is for different sizes) of Marriner's Heritage 3-ply. 3-ply is supposed to get about 8 sts per inch on US size 3 needles, which is fingering weight. I then looked in the Ann Budd Handy Book of Sweater Patterns and for a size 40 with a gauge of 7 sts per inch, it says you need 2000 yds. (There was nothing with a gauge of 8 sts, but that's close enough to get a general idea.) Then I looked at the yardage you get for 100g for sock yarn, and it's about 400 - 450. That would be about 5 skeins to get 2000 yds, which is 500g, which is about 17.5 oz. Which doesn't work out to 10 oz, but I think if you're getting gauge, then I wouldn't worry about that (and the finer the spin on the yarn, the more yds you'll get per oz). So I would get a skein or 2 extra to be very sure, especially because of the 7st vs. 8 st issue, and if they're left over, make matching socks
The one other thing I'd do is to see if 8 sts per inch makes sense in the pattern-- take the number of sts you're supposed to have across the bust and divide them by 8-- that will be the number of inches in the finished sweater, and so if that comes to a reasonable number, you're good.