yeah they are the right parts to get. so you can still hear the little motor whiring when you turn it on and off?
I've done lots of vt - vy ones, it must be pretty much the same though
one you get access to the aerial unit, it will be easier to take it out to get the old mast and rope out. there should be a little screw cap thing that goes where the mast goes into the motor, take that off and you should be able to pull the old mast and rope out, it can take a bit of force though. sometimes the old rope has broken and is stuck in the motor but you can take them apart to get to it.
if you dont have to take it apart, to get the new one back in you need to have power to it still, turn the radio off and when the motor is going feed the rope in and it should grab it and pull it in, put that little screw cap back on and turn it on and off a few times to make sure it fully goes up and down (even if its not all the way down when you start, it evens itself out.)
then just fit the whole thing back up into the guard.