What exactly is the activation tool? l have the procedure l just need the tool to do it, is it just the reset tool for sale by the dealer or is another possibly more costly tool?

You should take it to a garage to have it done. This only needs to be done if ur car can tell u which tire is low. If u have a low tire light that comes on & does not tell u where the tire is located on the vehicle then u will not need to relearn the sensors. If u got one replaced they should have did it. But if ur car does tell u where the low tire is then u only need to relearn the sensor when u get a tire rotation. so the tool will cost more then the cost to take it to a shop.