Hi Iain,
Your problem is simple. The L29YC is an extended nose type, where the centre contact stands out further and thus the earth contact as well. Champion part numbers with a Y in them denote extended nose. The C denotes copper core. You need to go back to a conventional style plug, simple as that!
I wish I'd checked your replies a few days ago! I've just bought and installed a new plug and realised the 'extended projection' of the old one after comparing. D'oh. All sorted now, running loads better and without any copper washers/valve contact.
