SA Surcharge: Relationship to Interest
TMA70/S59C only applies to the 2009-10 tax year, and any previous tax year.
See CH155000+ for 2010-11 and subsequent tax years
S59C surcharges are a separate matter entirely from S86 interest.
Interest (EM4000+) is simply restitution for loss of use of money over time, like any commercial interest. Surcharges are penalties for failing to pay on time.
Surcharges are payable according to their own rules where they arise and are in addition to interest.
The two topics should be kept entirely separate if confusion is to be avoided.