mike youre spot on.
i called up Max at Heritage MG Parts today (they also deal with minis) and he said that this saucer thing is extremely important and may be the source of my problem.
ive yet to get it (in 30minutes) and your explanation sounds logical.

we will tune it while we're at it. (our definition os tune is just adjusting the carby fuel mixture and idling speed... is there more to it?

if all else fails, ill have to get someone round to look at the timing. Max and dad says this is quite unlikely since its electronic ignition (significance?)

thank you all

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forever chasing clearer skies...

I dunno about "extremely important" half the cars just drive with an open breader hose going under the car (I've tried but it felt like a gas chamber). I think that until the engine has been running for awhile and its temperature and pressure rise this does not play much significance. As I said its only a valve/diafragm with 4 moving parts inside.
Just make sure you seal the hose to the manifold if you test without it.

If you have elec. ign than you wont have any points to adjust. You still need your timing set correctly but it just wont have to be readjusted for very long periods. (Doc correct me if I'm wrong).

What air filter are you using (its no use adjusting mixture with the filter off if it will get much richer once its on).

Commonly called a `Smiths valve' ('coz they made it...) it's a PCV valve. On later cars here with HS4 carb it was replaced by a connection into the side of the carb.

The pinhole you can see in the cover is to allow air pressure to get to the top of the diaphragm and act against the spring and vacuum below it.

If diaphragm gets perished or holed you can still get new ones. $10 or so.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.