In MG Enthusiast Feb 09 article talked about the plenum chamber valve that's under the bonnet with the pollen filter. The article talked about an uprated valve " Black Olive" by steve.
That's probably in the current (May) issue of MGE where I introduced old 206 (no, not the Peugeot!) to the readership. Haven't seen it yet as the camel train from England hasn't reached Oz, but Steve and the Collective get a plug...