In my experience within the UK graduate market I find that recruiters for larger companies usually give very frank and honest reasons for not hiring you.
You're in the UK? Well, that's the problem I'm afraid. People in the UK are not generally very accepting of startup failure, and will often count it against you.
I moved from the UK to San Francisco a few years ago, and everything is very different. An attempted startup, whether successful or not, is almost always seen as a good thing here.
I've recruited in the UK grad developer market, and I can tell you there aren't anywhere enough barely competent programmers on the market to make a failed startup background a problem. Most people applying for graduate developer jobs struggle to even write basic code and lack any real fundamental understanding of the languages they use.
I moved from the UK to San Francisco a few years ago, and everything is very different. An attempted startup, whether successful or not, is almost always seen as a good thing here.