Thanks mate, had tea and a beer tonight!
I've been trying to get the old Mocal oil cooler off, the block to the engine has a nut in the middle (taken the oil filter off) which isn't as big as a 30mm, but is bigger than a 27mm. I bought a 28mm socket, but surprise surprise, it is also too small!
Aside from buying a 29mm socket I don't know what else to try, without removing the plenum to be able to see properly or have more space for an adjustable, I'm stuck.
I've checked online for any info on Mocal oil coolers for an R33 GTR and found bugger all of any use! I don't want to refit the old cooler as I need this one off to fit the Nismo cooler and Greddy oil filter relocation kit when it arrives.