bottom bracket replacement

You have to figure out what is moving in relation to what.
If the cranks are moving in relation to each other, then try the crank bolt(s).
If both cranks are fixed in relation to each other, but moving (in synch) in relation to the frame, then it's the BB that needs attention.

Dabac is right on the money.

Try removing the chain from the chain rings and using your hand to try to shake and induce movement into the crank assembly. You've got to figure out where the movement you're experiencing is originating. Rotation the crank without the chain will also allow a rough assessment of the lubrication/contamination of the bearings if you feel or hear roughness or grit, odd noises, etc.

Check tightness of the chain ring mounting bolts and the crank arm mounting bolts. It they are tight and play is still detected, it's likely the bottom bracket bearings need adjusted to remove excessive clearance or possibly replaced. The crank axle and/or the bearing cups may also be worn.

thanks lads,i removed cranks and bb and gave them a good clean. greased them and replaced them..seems to have worked a treat for now..