• non-contentious: you may charge interest on the whole or outstanding part of an unpaid bill from one month after delivery of the bill. This assumes written notice has been given to the client of the right to request a remuneration certificate and of the right to seek assessment of the bill by the court (presumably you include that in your standard terms and on the bill?);
• contentious: you can charge interest if (i) the right to charge interest has been expressly reserved in your original retainer agreement; or (ii) if the client has subsequently agreed to pay it in return for a contractual consideration; or (iii) if you sue the client and claim interest under s35A SCA 1981.

See note in [2007] LSG 8 February 35. © Practical Lawyer