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over the course of a about a month, it's generated a little poo (for want of better word) that sits on top of where the crack is - a sludgy brown semi-solid perhaps the size of a thumbnail.
@user69121 - thanks! i didn't even realise earlier that this bit was a joint/each vertical section (do they have a name?) was an individual connected part - assumed the whole thing was one big hunk of metal..! in many ways, good to know as that hopefully means the metal itself isn't pitted and full of holes (touch wood)
hvsds-
The sections are joined* by threaded nipples, so your leak is probably having to track down the screw threads and past the fibre washer.
Each section has a Left hand thread one side and Right hand thread on the other. So pairing a LH thread with a RH thread allows the nipple to draw two sections together as it's rotated.
Getting the plugs out of the top would show which end/sides are left/right thread. You can measure the thread size required and order some parts.
https://www.castrads.com/uk/product-category/spares-accessories/
(*rare, but some rads just have a long tie rod right through, with Bushes to align the sections and seals )
user69121
Is it leaking?