My CBR400 NC23 recently started to leak oil from the casting indent in the left side of the cylinder head, can anyone offer any advice to its cause/repair. Thank you

small hole on the RHS of the bike?

if so.. this is only the rocker cover gasket. so not the end of the world.
However a PIG to fit a new one.. advice you get a mate to hold the radiator out the way & help get the rocker cover back in with a new gasket held in place with gasket seal.

Thanks will get a gasket and give it a go.

getting the rocket cover off 7 old gasket out was kinda easy..
tank off, air filter off, coil packs off.. HT leads off BUT make sure you take note of which one goes to which coil pack/plug...
LEAVE THE PLUGS IN.. stops crap falling in the calendar head.
unbolt radiator but dont need to drain it hence a mate to hold it as far forward as they can.
rocker cover bolts off.. tap around the cover with a rubber mallet...

clean the cover when you do get it off.. use gasket seal to seat the new seal, leave it for an hour to "set" then slowly & very carefully feed the cover back in... took me 3 tries of catching the new seal on the cams which pulled it out of the seal seat..!

Ive had the rocker cover off a few times but ive always removed the radiator, never had any trouble getting it back on, by the sounds of the above posts leaving the radiator on seems alot more hassle than its worth.

wasnt too much hassle with my mate helping.

BUT.. just learnt samat today. my choke cable wouldnt pull out as far as it used to.. oiled it.. nowt
got a new one & went to fit it today... to find the choke slide bar was being blocked by a breather pipe i MOVED while i was doing the rocker cover gasket...

try to put stuff back where it came from...