This is a completely superfluous double-dip (which I only have because it is included with the "Classic Collection", where it is even more useless than on its own).
The "film" is a collection of scenes taken from the various Sharpe episodes, taken out of context and linked into a new one by Cooper's narrative. Not scenes cut out of the other films, no new information - nothing about why Cooper disappeared (I mean, if he died, he couldn't sit there with a drink telling us Sharpe's career, could he?), nothing about where Sharpe is now. Just scenes you have already seen when you've seen all the Sharpe episodes (and Legend was made before Sharpe's Indian adventure, so nothing about them either).
So, if you have a complete collection of Sharpe's, you don't need Legend. If you have not yet seen the episodes, do yourself a favour and do NOT watch Legend - it's full of spoilers. But do get the episodes and watch them...
That said, Legend is pleasant enough to watch - but the episodes are better.