I'd just like to point out that this thread was made in 2010 so I'm sure the OP doesn't need any advice about this anymore (and hopefully they've graduated!).
However, anyone looking at this thread for information, I'd like to share something another student shared about this issue with the OU percentages.
If one person has £20 and someone else has $28, they are of the same monetary value, but they're different numbers. The OU is no harder or easier than a traditional university, just their percentages and scores are done slightly differently which results in a different percentage threshold.