If you are sure you can either accept or reject it on the spot, (final answer ??) or defer it by saying you would like to think about it for a couple of days. Then phone them up saying that you really like the job etc, but you are really looking for that extra 5k, and if it were available you would be happy to say yes. That gives you a second chance to attempt negotiation which for any of a number of reasons may succeed where it didnt the first time...

As an aside employers usually have a band for the job spec, and you need to find out what this is, and ensure you are at the top of it at least...

dave