Lovely plants [USER=55]@NigelJ[/USER] 
I think Tetters has got it, there's a monstrose subulata that's very slow growing. I've got a small one that's at least 6 years old and only about 6 inches tall, also inherited one that's much older but still only a few feet high.
The normal A. subulata is huge and grows a foot a year quite happily 
This "log" is a normal one on it's side, it was pruned the year before and all the shoots are new growth.
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This is the "Monstrose" version, which can throw out reversions like the one above occasionally.
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The other possiblity is that there's not enough nutrient in the pot for yours to grow to it's full size, my Brother has got one in a tiny pot that he never feeds and it's eaten itself to produce a new shoot
