Thetford Flush Tank grunge

Hi All, 

Caravan serviced before trip. At first halt electric flush didn't work. On arrival at end destination removed pump from cistern and rotated the pump impeller. Works fine. However, removal of pump revealed hideous slimy grey grunge all over pump and tubes and I daresay body of tank. Drain off was slow so cleared that with application of fine jet on air awning hand pump. Only thing I can get here in France is Bicarb of Soda which has been left in tank 24hrs. Not wholly convinced this will have done much. Any thoughts please??

PS sure I caused with over enthusiastic use of pink fluid - more is better .... NOT