seized nut connecting ball valve

[annabella]

Hi everyone,
Am a plumbing virgin, trying to replace the whole ball valve mechanism as the old one is crumbling. It looks easy enough on the instructions in the pack EXCEPT I cannot for the life of me undo the plastic nut thingy that tightens against the underside of the cistern (it's bottom filling type). My fingers arent very strong and the nut is bigger than any spanner I can find. Is there a special tool i need or am i missing something obvious?
THANKS!

 

[JohnD]

bigger spanner

discover the joys of tool shopping

better than shoes

 

[namsag]

PAIR of grips or just hacksaw through nut take ten second

 

[annabella]

Thanks guys, will try these ideas

 

[Agile]

Remember to turn off the water first.

I wonder how she manages to post an hour in advance?

Tony