Building on the extreme simplicity and ease of use given by Tips and tricks HQ I needed to add support for multiple currencies for checkout and display.
The approach taken is to extend the plugin to have an extra layer that simply converts the default currency into an alternative currency for display and payments (when required).
The rates are automatically taken from the ECB (http://www.ecb.europa.eu/stats/eurofxref/eurofxref-daily.xml) every 24 hours and cached.