Oil pump chain rattle

Evening guys,
I've had a rattle that starts from 5k onward so yesterday i dropped the sump to have a little look at the bottom end and everything looked to be alright except a few bits of broken black plastic in the sump and pick up pipe.
i had a feel of the pump chain and it felt quite tight then today the chain on the opposite side to the tensioner felt really slack, i could move it side to side by about 10mm but i hadn't touched anything so thought nothing of it and re sealed the sump etc. went to start it up and now it rattles like mad just on idle and is differently coming from that chain. so i'm guessing the tensioner has had it.
Is it possible to replace it with the engine still in the car?
and has anyone els had this problem?

ok, i assumed so. is the crank pulley accessible from under the car, im assuming its torqued on and limited space would be nearly impossible to remove.
have you removed one whilst engine still in situ before?