Have you done a compression test?
And is the engine the original 848cc or is it a rebuilt or donor engine from something else?

_________________
Respect mine and I'll respect yours.

For a sanity check, you can change the plugs around the head to see if the fault follows the plugs, you can also do this for the leads.

Clean your distributor cap out, give it a good dusting out, checking also for moisture.

Plugs on the head can give good spark while out, but once they're under compression it is harder to jump the gap by a large margin.

_________________
SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

Try a new set of plugs....and make sure all yours leads are delivering spark to the spark plugs

_________________
"Show me the Mini!"

mine did the same thing checked the gaps on the rocker arms and found out i had a few rockers chewing their way through the rocker rod.