Which tax offce?

I submit SA returns online.   In updating software for 2010 returns, the software company say that I must show on each return the relevant service office of HMRC who deals with that client.

Most client's are self employed and were previousdly dealt with by the tax office most local to them.   Following the HMRC brainwave a few years ago to tax all private prensions on PAYE and issue a code for every pension, the PAYE office suddenly became the main office.   By way of an example I have one client who received tax codes from three different tax offices for 2010/11 to tax pensions from three different companies - neither of which are the original tax office.

I phoned HMRC on this client and was told that the 'service' office was, in fact, different from all three coding offices, as one dealt with post for his service office.

Most, but I suspect, not all clients received a notice to file a return in April.   However, as they know I have filed online for sevreral years most have probably destroyed these on the basis that they were not required.

My query, therefore, is - how can I easily find out which is the service office for each client, and, as I submit online, does it matter anyway?   In the good old days of client information sheets twice a year I could have got it from them.