Whenever people call the acting in Classic Who “rubbish” or complain about the bad effects, I just laugh. The explosion here was much bigger than planned. It nearly burnt Sylvester McCoy’s clothes off, but he kept walking, staying in character, because he knew they couldn’t redo this scene. That’s dedication. *tips off her hat*
McCoy also saved the actress who played Mel life. The tank that was filling with water had started to bend and he ordered the men who were filling it up with water to pull her out. She just escaped when the glass cage collapsed. She would have died otherwise.