HMRC payment plans - anyone any experience?

I have a client who has an outstanding SA tax bill of some £10,000. The clients business failed and they made payment plans with all creditors including HMRC. The client is now working full time as an employee.

HMRC agreed a payment plan last year whereby the client could repay the debt over 15 years based on all they could afford to pay and this was confirmed in writing. All agreed payments have been paid on the due dates and I have no doubts whatsoever that the client will pay the debt in full.  

They are now harassing the client and suggesting the payment plan must be reviewed and may be cancelled.

It seems a very unusual way to deal with someone to suddenly re-open the case and seek to amend an agreed plan. 

Its causing the client a huge amount of stress - any suggestions?

I am considering a complaint but am uncertain of the best way forward.