Applying for postdoc fellowship vs applying for lectureship I finished my PhD 2.5 years ago (Computer Science), and have since been working as a post-doc at a top university. I've decided that I want to pursue an academic career, and I'm trying to decide what step to take next. I'm at the stage where I could start applying for lectureships (assistant professor), and there's a decent chance that I would be successful -- at some university at least, even if not the very top ones.

However, the other option, which I have been recommended to go for, is to apply for a post-doctoral research fellowship. These prestigious fellowships are sponsored by independent funding bodies, for people with up to 3 years experience since their PhD, and seem to be designed for the very best academics. (If you are familiar with the UK, then I have the Royal Academy and EPSRC fellowships in mind.) However, my worry is that because these last for up to 5 years, then by completion, I will then have had around 8 years of post-doctoral experience. This seems a very long time before I apply for my first lectureship -- and if I started applying for lectureships now, I could have risen to senior lecturer (associate professor) by then.

So, my question is, if I apply for one of these fellowships and am successful, why would it actually be a positive career move for me? It would give me time to focus on my research which is great, but for my long-term career, wouldn't the delay in taking up my first lectureship position ultimately have a negative impact?

Thank for any advice!
It is never too early to apply for permanent positions in academia. I would simultaneously apply for both the fellowships and lecturer positions. Once you get a 5 years fellowships (and I assume you are talking about the 5 years advanced postdoc fellowships), you can negotiate a permanent lecturer position. From the university's side, they get a person who has already been successful in obtaining a prestigious fellowship without any significant financial burden for the first 5 years (because the fellowship would pay most of your salary)! From the candidate's side, he\she gets a permanent position without having to do significant teaching or administrators duties in the first 5 years. I have seen many of my colleagues successfully going this route. These may or may not apply to Oxford-Cambridge for whom such a fellowship is not enough to offer a permanent position. But I also know people who got their fellowships hosted by Oxford-Cambridge, and then negotiated a permanent position with another university in the mid-way during their fellowship, i.e., porting their fellowships to another institution which offered them permanent positions.
In fact, I view this as a great way to transition from a postdoc to permanent position. 

I wish such an option was possible in the USA. In the USA, such 4-5 years 'fellowships'(CAREER awards) can be applied only after you secure a tenure-track position at a university.