Hi Mark,
If I was you I would set up a limited company, it will cost you about £20 if you do it yourself online, or an accountant will charge you around £150.
After that just open a bank account for the company and start sending those invoices and fill up that account. This will not affect your dole money(tax credits im assuming) as those are currently based purely on your income, so if you don't take a wage from your company its all good.
Once you company account is full or cash, then you take dividends or a wage and your tax credits at this stage will be affected.
If you go down the Umbrella route however they will deal with you through PAYE so it will affect your benefits, give this company a call to double check: [broken link removed]