A little update that may be of use to others...
Finally got a reply from my provider today. Hardly touched any of my requests for information, but did say latest (and totally unexpected) demand was based on P60 2017. The only way it could be wrongly calculated was if I had deductions such as pension etc., which are not included.
For me to owe £600, I would need to earn an average of £1915 net per month, but I very rarely get anywhere near to that income, even when working Christmas Day, Boxing Day and New Year..! However, I pay roughly £370 a month directly into a pension which is deducted at source, and hence not shown on P60.
My provider, despite numerous requests, will not itemise the extra months of payments added to the end of my plan. They just say "arrears solution for unpaid PPI and undeclared earnings". I've tried to challenge these charges with little response, but now I'm thinking these are probably related to my pension contributions as well.
With only 5 days left until my IVA goes into a third missed payment, and pending failure - it would have been nice if my provider had been at least a little bit more helpful a little bit earlier