After some recent plumbing work to replace ball valve in loft , now when I turn on the bathroom tap the toilet stops filling with water any ideas?

is your tap and toilet tank fed or do they connect to the mains also if you could be more specific ie original problem with loft ball valve then i could give you a better diagnosis

Is the supply to the bathroom taps and Wc gravity fed (from cold water tank) or mains water ?

bloody hell Darrell that was uncanny - think that must be the closest to a simultaenous reply I've ever come across !

great minds think alike dude