I've been told by my British colleagues that I should go to the tax man. i think that's what the British call the National Insurance?

I've been paying taxes automatically from my salary but someone told me, I should go to the "tax man" at the end of the tax year. They also advised me to get a tax accountant to sort things out. I'm driving a car and the petrol is paid for the company (private or business) and I was told that I am paying tax for the petrol but I should get refunded for the tax I paid for business... how do I do that? Im doing around 12000 miles a year so that should be quite a lot. (??)

So, I was also told that if I dont do nothing I dont get the money back.

furthermore, if I get a tax advisor, I will pay him around more or less £200 and he takes care of everything. is that rate reasonable? If i get £1000 refund, i think it's worth the hassle.

opinions anyone?

Thanks...