We've just inherited a compost bin with our new house which was full to bursting but I'm not sure that the contents is proper compost. I've gone in through the flap at the bottom of the bin and the compost is more like a sludgy clay - definitely not crumbly or looking anything like commercial compost. There appear to be large hard bits in it - no idea what and it looks like the previous owners emptied old pot plants etc. into it as there are bits of plastic and roots in it too.
Does this sound right or should I somehow attempt to get rid of the contents and start from scratch?