@dafzthomas - I think it may be more to do with timing than anything else. The world and their wives are using wireless for everything these days, from toasters to phones, and as a result the WiFi band can be quite congested.
I suggest that you download an network diagnostics tools for your phone so you can see what is happening around you. I can't recommend an App as I don't even have a mobile phone never mind an iPhone. But someone else will come along and suggest something.
As an aside, have you separated your 2.4 from your 5.0GHz bands within your router? This can make a big difference in WiFi performance and is well worth doing if you've not done so already.
