In a nutshell your score will improve by a few points when the completion certificate is issued and by quite a few more when the IVA and defaults disappear on the 6th anniversaries. From then on it's a case of tweaking by the sensible use of credit and building up a history.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014 http://foggy.blogs.iva.co.uk