Can anyone tell me what pulley this is that disintegrated on my HDi last night?

[West]

Ah so where the hole is, is that just a cover? I thought it was the crank pulley, I had a quick browse through my Haines manual last night but that was at like 1AM and I was pretty tired and couldn't come to a conclusion. I work away five days of the week, so I will have to get on the case next weekend and decide if its worth repairing or not, as a i need a new rear axle to get it through the next MOT due to worn rear end bushes. I think the differential might be on the way out too as the the steering wheel wobbles pretty bad when coming on and off the throttle. Its not looking good haha.