So, I tried this ... Got a LOT of carbon out... But as I didn't have a "shop" hoover, ended up just scraping off loads, getting it out using a screw/screwdriver/finger.
Put it all back together, have primed for a while... Double checked electrical connectors, vacuum hoses etc, all seems fine.
But car won't start. Just turns over.
About to get the computer connected (managed to get techstream working today) but if carbon has entered the block, how bad is that?