Your salary is subject to income tax and employee national insurance contributions. In addition, your employer pays employer national insurance contributions. If you go here
and enter your gross salary and put a 0T tax code in the relevant box, it will tell you how much tax and NIC is paid by you and the company (NIC only). This is on the assumption that your personal allowance can be used against your self-employment income. You can also put in a tax code of 944L if your personal allowance is used against your salary.
You could sacrifice salary and have the money sacrificed put into a pension plan. However, you are limited to the lesser of £50,000 and 100% of your earnings being put into a pension. Both personal and employer contributions count towards that total. You can also use the unused allowance from the three previous tax years.
As you have self-employment income of £15,000, you could use your personal tax allowance against that and take all your income from the company as a dividend which would be treated as basic rate tax paid. Dividends are not subject to national insurance contributions. In theory, provided the company has sufficient post tax profits, you can take up to £37,678.50 in a net of 10% notional tax credit dividend in the 2013/14 tax year and pay no further personal tax but that would be on the basis that you had no other income which you do. Deduct £15,000 from £41,865 (the sum of the basic rate tax band and the personal allowance) and you could draw up to £24,178.50 net of a notional 10% tax credit and pay no tax on it. Take a look here
for information on dividends and tax.
Whereas salary is a tax deductible expense for the company, dividends are not so by taking dividends as opposed to salary you are effectively increasing the corporation tax liability of the company. If there is more than one shareholder, dividends have to be paid at the same rate to all of them unless there are different share types or dividend waivers, the use of which has to be carefully managed.
You really need to have a meeting with your co-director/shareholders and your accountant to discuss the matter as there is a balance to be struck between efficient salary and dividend planning. The tax effects on both sides (employee and company) also have to be considered.
I hope this helps but let me know if you have any further questions.