Older "insert type" oil-filter removal?

I've got one of these on my SXE and I haven;t ever been able to remove it. Are there any tricks that can be shared for shifting it? It feels like the bolt in the centre should be a 16mm but looks like it's been rounded off by someone also trying to remove it.
oil filter.jpg
I've purchased a chain tool that was recomened by an auto factors for oil filters but not sure what I am doing here, I can see how this would help with a "screw on" type but not sure what I'd do with it in this case.
chain tool thing.jpg
(my blue Mera and the new Prim both have screw on type, only the SXE has this annoyance)
 
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