Thanks for the comments - in response to Dom, I've done my own stuff, but it's been a few levels down of where I want to be with my art, e.g. my values were always a weak point as I either ended up with a page full of mid-tone or I'd shade areas in isolation, not seeing the picture as a whole, so the values wouldn't work properly across the piece.
By replicating others' work and comparing, I've been able to recognise this and make improvements - as I tighten up some other areas I'll be adding more and more of my own stuff, but at the moment I'm still getting into things - I've only tended to draw every few months or so, so I need to set some time aside where I can just focus on art.