Astra G - Fuel Tank Mountings [Hatch,Saloon,Coupé,Convertible,Estate,Van (F07,F08,F35,F48,F67,F69,F70)]
18. SCREW,HEX.HD.,M8,FUEL TANK TO UNDERBODY - part # 13209785 (x4)
19. NUT,CLAMP,M8,FUEL TANK TO UNDERBODY - part # 9202469 (x4)
23. STRAP,TENSION,FUEL TANK SECURING,RH - part # 24408506
24. STRAP,TENSION,FUEL TANK SECURING,LH - part # 24408505
25. STRIP,DAMPER,FUEL TANK STRAP,RH - part # 90571734
26. STRIP,DAMPER,FUEL TANK STRAP,LH - part # 90571733
28. PLATE,PROTECTION,FUEL TANK - part # 24408878
Probably not a good idea to remove the fuel tank cover plate as it can become corroded around the fuel tank, so you will have to ascertain whether one of the straps has become damaged or just a nut and bolt is required to resecure the cover.