The government intends to beef up HMRC's powers to recover tax debts directly from taxpayers' bank and building society accounts, including ISAs. The new measure will apply, to taxpayers who owe at least £1,000 and have been requested on several occasions by HMRC to pay. A minimum aggregate balance of £5,000 will have to be left across all of the taxpayers' accounts. including ISAs, after the debt has been recovered. Consultation on the implementation of this measure is ongoing.
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Also be aware that HMRC is prone to "mistakes" and once he has obtained your money by fair means or foul it could be the devils own job to get the money back.
Obviously to prevent this happening please ensure that all tax is paid in good time or ensure that your bank balances do not hold funds of more than £5,000 across all the accounts.
I very much doubt that HMRC can touch funds that are held jointly with your spouse of partner.