sorry to dissapoint you on the guesswork
the sparks was back promptly the next morn.
apologisied profusely "schoolboy error on my behalf" was one of his comments
anyway it was a live which seemed to be the prob. (a loop of 10sq coming from the bottom of the rcbo) the live feed i presume. took him all of 5mins to fix.
getting a look in side the cu i could see that the rcbo is on ots own circuit, it just physically happens to be on the right of most everything else.
probably because he arrived with a cu with most of the components allready installed, except the rcbo, and the mcb for the shed which was wired into a mini enclosure by me a couple of years ago.
i did also mention the cooker circuit - rcd. he said it was on account of it being a fixed appliance, hard-wired in , or something along those lines, he even offered to show me the relevant regulations.
anyway if anyone is still dying of curiosity the get a butchers inside of the unit , just say and i will try to get get a pic on here tomorrow,
thanks again lads
great forum this, couldnt have done my bathroom without all the great advice i have gotten in all of the various sections