Top slicing calculation not showing correctly in calculation

Hi, I have a chargeable event certificate from liquidating a life insurance policy of 18 years. The gain was circa £11,000 and takes me into the higher rate tax band due to a salary of £44,000. On the form I have entered the amount with tax treated as paid as £11,000 as 20% basic rate has already been taken as per the certificate. However the top slicing relief is only ~£750 leaving a remaining £1,000 bill. As I had £6,000 of margin before I entered the higher tax bracket, surely just 3 years of top slicing would be sufficient, and certainly 18 years should be enough?

Hi
Customer Forum advisers can provide general advice and guidance on Chargeable Events, but we are unable to answer questions which relate to specific calculations.
Please call our Self Assessment helpline and ask to speak  to a Technical Adviser.
Top slicing relief
Thankyou.
Regards.