Client WANTS to make payments on account

My client is one half of a husband / wife partnership, and when his 2010/11 tax return was filed the previous accountant checked the box to have tax collected through his tax code if less than £2,000 tax was owed.  This was in error as my client has a strong preference to pay his taxes through the normal payment on accounts and final balancing payment.  The client paid the final balancing payment from 2010/11 which now shows as an overpayment on his HMRC statement.  The wife's tax return was done according to their instructions without the box being ticked, so she is still making payments on account.

The client has contacted HMRC by phone and asked if he can revert to paying the tax as normal, instead of having the 2010/11 balancing payment collected via his tax code (he has some pension income), and then being hit by a large payment for 2011/12 when the tax return is filed, but was told he couldn't do so.

Any ideas?  Is it worth writing to HMRC to formalise the request?  Its so unusual to have a client be upset because they haven't been allowed to pay tax!