Thanks guys you guys are more help then anyone else I'll call them asap
You can inform them online if you prefer. You'll need to tell them that you work as a sub contractor as that falls under the CIS (Construction Industry Scheme) rules. Under these rules your boss should be deducting your tax before paying you. You then declare this when you complete your tax return.
It's not as scary as it sounds. Good record keeping and the advice of an accountant should be all you need (plus submitting your tax return on time!).