Just to update everyone. Design tool trials are underway. Both Mockflow and Balsamiq look like good options. Tony has discounted UX Pin on the basis of performance.

I have reached out to the guys at Mockflow to see if they provide discounts or drop license fees for open source projects.

Balsamiq do this as per: http://support.balsamiq.com/customer/portal/articles/105924

Thanks
Sven