This was not an issue with plugins - it is not a bug but it is to do with Thesis 2 and tracking form submission with google analytics.
I noticed an error in debug - something to do with the page tracker code. This is an additional bit of code added at the bottom of my contact form in wordpress admin to allow google analytics to provide data on successful form submissions. My old code was as follows;
on_sent_ok: "pageTracker._trackPageview('/contact-submit.html');"
Looks like I had an old version of the code.
See here for instructions on the new code which I added (remember to add the goal from within google analytics as well as add the code in the wordpress admin);
http://wordpress.org/support/topic/plugin-contact-form-7-virtual-page-view-tracking-code-interrupts-response-messages?replies=7
On further testing I was still having trouble with it. Then I realised that when you use thesis' inbuild google analytics tracking feature, when logged into the admin thesis does not generate the google code on the front end of the website. This stops data being collected on activity by developers and site owners which is good. But it also stops the code working correctly when you test the forms and generates a js debug message.
When I opened a different browser and tested the forms worked fine - log out of the admin and test and it works fine.