Ok - to the OP. You've probably done this already either on your own or w/Apple but I ran across it inadvertently and my Link Speed is now 300mb
In your airport utility make sure that the radio mode is set to 802.11n only - that's what got my speed up.
I'd already done that - but thanks for the suggestion.
as I mentioned before the country in the setup seems to be also a big part. I configured it new with a different country than Germany (I took Italy) and now I´m able to get 300 MBit with the 5 GHz and 130 MBit with the 2.4 GHz. With the Germany setting also 5 GHz has been only 130 MBit.
Best regards - Christoph
You're a genius! This has sorted it. For whatever reason, the UK doesn't support the wider (faster) operation despite the post above to that effect. Even the 'genius' in the Apple store didn't suggest this!! Changed to Italy and now my network flies!