Cheers Dave. I may be clutching at straws but I was trying to think what I've changed that may have caused a problem, and the only thing was fitting the catch can. I've hit full boost a few times previously and not had the dipstick/oil spray. The hoses are a bit of a bodge (just what I had in the garage/proper ones on their way) but step down to 12mm from 18mm, and I wondered if the can/filter is too restrictive for the PCV gasses (perhaps more suited to an n/a application)? I've removed them for now and just left the stock can venting to atmos, so shall see if any difference when I go out later. The only reason for doing the can install was, having cleaned the usual gunk out of the intake system, I didn't want it to return.
I'm not sure where my compression tester is but will obviously try to do one asap, even though I know it doesn't always show the problem with ringlands.