Saying if they had a competent leader is backwards thinking. They don't have a strong leader because they don't really have an identity. They are split between two big groups (Blairites-Brownites) both of whom are antiquated and out of touch. Their core identity - labour - is completely disconnected with them. The Tories have them in Southern England and the SNP in Scotland. Tony fucked them up big time.
I'm not saying if they had a strong leader, I'm saying if they had a semi competent one. Ed Milliband is by all accounts a relatively bright guy who has had in the past* some relatively interesting things to say on a few issues, but as a leader of a political party he completely fails the retail test which largely is the most important facet as UK politics get more and more presidential.
Labour could have a leader with a backbone made out of soggy cardboard struggling to straddle the divides between all various factions in the Labour party without so much as clue in terms of policy and they'd still win if that same leader could simply sound like someone people could relate with while sounding like he had a semblance of a clue. Ed doesn't.
For the opposition it's not so much what their policies are (as long as their not too out there, although the rise of UKIP makes me doubt even that now) but how the differentiate themselves from the incumbents. Oppositions don't get voted in, Governments get voted out, all the opposition has to do is not be too dickish and sound competent. Milliband may not be a dick (I don't know) but he doesn't sound well human never mind competent.
*unfortunately for Ed his interesting things remain in the position papers he wrote ages ago and never really get promoted now he's actually in charge of policy.