The accident happened when an underground tank containing oil and fuel was being removed. Polden used a petrol-driven saw to cut into the tank, which resulted in a fireball in which he received serious burns.
Astin pleaded guilty to breaching Section 3(2) of the Health and Safety at Work Etc Act 1974, which requires self-employed persons to ensure workers not directly in their employment are not exposed to health and safety risks.
Health and Safety Executive inspector Adele Davies said Astin had fallen short of his responsibilities.
She said: "This was a foreseeable and avoidable incident. The removal of contaminated tanks is clearly a specialist activity, and a specialist company should have been engaged to carry out the work."