To be honest, I hadn't noticed the second round of Party Politicals on the TV (and if I've missed the Labour or Conservative broadcasts, please let me know). The Lib Dems second broadcast is a little more sharp than the first. It begins by pointing out problems with the policies of both Labour and Tory, then goes on to present the LD view on various issues.
In my view, this worked much better than the previous offering as a party political. Yes, it had a bit of negative campaigning to it, but this was balanced with the positive.