Slow cranking on an 02 1.0 vvti Yaris

[Duberry]

My daughter's 2nd hand 02 Yaris, purchased in 2006, has always been a very slow cranking car, it has always started and we have just accepted it as a querk of the car.

Earlier this year it seemed to be turning over even more slowly so I changed the original battery for another Bosch unit. This improved the cranking, but only marginally.

However, in the past few weeks its cranking seems to have got even slower to the point where we don't think it is going to start. So at the weekend I intend to change the starter motor.

My question is, has anyone else suffered slow cranking and did changing the starter motor fix it? Is this an easy job to do? Also, what do the various relays - integration, starter, ignition etc do and could any of these be responsible for the slowdown in cranking?