UK Tax Questions? Ask a UK Tax Advisor for answers ASAP
The reason the 0TX code is being operated is probably because the pension payer wasn't give your wife's P45 that she should have received from her employer when she retired.
All your wife needs to do is to call HMRC on the number here and have them review her tax code and change it if necessary which is almost certaibnly the case while she is on the phone to them. The pension payer will be notified of the new tax code electronically.
I hope this helps but let me know if you have any further questions.