Someone told me i didnt need to declare this as i was working for a dutch company, is this correct?
This sort of advice is usually the most attractive, and often the least accurate. You are British, working in the UK and (I assume) living in the UK. So you are liable to UK tax on your worldwide income and gains.
You need to establish whether you are employed or self employed. Remember that it is based on the terms and conditions of the engagement, not the choice of the worker or engager - even if they come to mutual agreement. If you are self employed then you simply complete a tax return each year.
If employed ... my memory is rusty, but I think that the Dutch company is liable to operate PAYE on your earnings if it has a UK tax presence. Or something like that. If not, then you may be required to operate PAYE and account for and pay over the tax and NICs. Someone will know better than me ... I have looked in the Employment Income Manual and Employment Procedures Manual but can't find what I want.