Load default currency if cart is not available

Further to the problem with dispatching orders I have added functionality to the format_currency() helper to load the default currency if it can't find the right one.

I am also having problems with the dispatch email not showing £ signs in the totals

![screen shot 2013-07-24 at 13 38 17](https://f.cloud.github.com/assets/255807/848381/f8eb56fc-f45d-11e2-85d1-a1e3c26cfda6.png)

However the £ signs are appearing fine in the confirmation and the admin notification emails, the only thing I have noticed is that $format is set to false for the dispatch email and true for all the others but I can't track down why this is happening, any ideas?

![screen shot 2013-07-24 at 13 43 10](https://f.cloud.github.com/assets/255807/848395/a2918c30-f45e-11e2-9110-e0f6a955e4c8.png)