Ok, so, I've found my vacuum leak (a hole on the 'octopus' cam cover breather pipe).
Gets a shiny silicone one from forge, has a look (thinks - this looks simples). Go to push it on the plastic downpipe (no idea what it is called) and the tip literally disintegrates in my hands.
I've pushed it on to what is left and it has certainly 'fixed' the boost leak but I definitely need a new one sharpish.
Any one any idea on a part name & no.???