update to this, did a compression test and cylinder 4 is down to 135psi, the others are between 150 and 155psi.
clearly a problem, but not so massive that i would have thought it would be kicking loads of oil out.
so i checked oil cooler and pressure tested for leaks and all was good.
cleaned breather pot and pipework and all is now clear (did have quite a bit of mayo in the pipes from hot oil/condensation)
got a bit of fumes coming out of breather filter.
maybe in need of an oil change possibly, been a while since the last one.
lost quite a bit of oil as it landed on exhaust and burnt nicely