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I need to get it off and it looks like I shall need ye olde crank puller.
Want to make sure I get the right one.

Looks like an XT crank to me.
Cleaning it might help 🤣 and a picture of the other side.
What BB is it on?
Not Shimano
There may be some markings on the inside of the crank arm under that protective layer of mud.
Don't you just need something like this that will work for square taper or Isis/octalink/exi. The adaptor makes it work with the hollow axles.
https://www.tredz.co.uk/.Ice-Toolz-Crank-Removal-Tool-Fits-8mm-Hex-Key_37707.htm
Just checked and it's a Suntour.
Can I retro fit self extracting bolts?
Was going to say suntour. Looks the same as the ones my son took off his voodoo bizango. That had a hollow tech BB. Used my usual crank extractor, with a plug for hollow tech BBs, like the one Nixie linked to above.
I've got an old square taper extractor.
Is that going to fit? From memory it's going to be too small
@zippykona got any 1/2 penny’s, from memory they are the same size as the adapter.
Yeah square tapper extractor won't work, will just go through the hole in the BB. You'll need one of these https://www.wiggle.co.uk/shimano-octalink-chainset-plug-tool
Stack of 5p's