Will I pay tax on my endowment policy

I have just received an endowment payout of £34,076 and my endowment was to pay £15000, although originally in 1987 the sum assured on the policy was £30,160. When we realised that there may be a shortfall we moved £15,000 to repayment. I have been paying interest only of £53 each month to my building society for the £15000, and full repayment on the other. The endowment was paid monthly for 25 years starting on 15th May 1987 at £43.00 per month. I guess I have a capital gain of £19,076? The difference between what I owe the building society and the matured sum? Do you know if I will have to pay any tax and if so what the % tax rate would be? I am a 40% tax payer and earn about 70K a year. The actual name on the policy is " Low cost endowment assurance with profit". The policy included life insurance. As I understand it the policy was "qualifying" as it was over 10 years and a fixed payment that ran its term.

Any help much appreciated