Yes and no is the answer. If you live in England or Wales, you'll either require a competent person to install it, or else you'll have to apply for a Building Notice via Building Control. For competent person in practice read OFTEC Registered Technician - see
http://www.oftec.org
As for the guy who came who came to visit you, yes there's a raft of rules and regulations governing the installation of oil storage tanks. For domestic installations, most regulations fall under Building Regulations and not Environmental Regulations. See:
http://www.tankdepot.co.uk/regulations.htm
http://www.tankdepot.co.uk/installation_guidel ines.htm
Also, practically every domestic oil tank should be bunded. To see whether it needs to be, check out the OFTEC Risk Assessment:
http://www.oftec.co.uk/publications/T14_Iss2_s ept_05.pdf
Reference DIY, who indeed is to know you if you've DIY'd it?
Well probably the tank manufacturer when they laugh at any warranty claim you may make; or your home insurers when they decline an insurance claim; perhaps a surveyor when you go to sell your home; or perhaps Building Control, when they wonder where your CD10 form is.