(I am interested to know what others think re level of penalties!)
Disclosure - he walks in off the street and makes a full and voluntary disclosure with no fear of discovery through any compliance activity. Abate by max 30% (includes the extra 10%).
Co-operation - if he produces all records, provides honest and prompt answers, attends a meeting if requested, makes an early and substantial payment on account and deals with all correspondence swiftly. Abate by max 40%.
Seriousness - difficult to say for sure. Worst case scenario is that several years are involved, that there was a deliberate failure to notify or submission of incorrect returns and that the amounts are high in absolute and relative terms. Max is 40%, but abate by 20%.
So net penalty might be as low as 10% - but a lot of that is down to a full disclosure and total co-operation.