As long as you have no equity to worry about and have had no breaks, missed payments or reductions your payments should end, as expected, in July -- but please check this with GT.
Around June time you will get your final review papers. Once that is completed your file will be passed to GT's completions team -- then I am afraid, it's a waiting game for the final certificate.
Has GT started looking into PPI yet ?
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014
Foggy explains what should happen. Unfortunately, it may well not be that smooth or simple. GTs actual current stance is upto 6 months after final payment to conduct and process a final review (during which stage your iva wont even be in closures) and then upto another 6 months once it goes to the closures team. So expect upto 12 months, and probably a minimum 6 months, for final completion after your final payment, though with a bit of luck things will have improved a bit by the time you make your final payment.