RATATATAT sound when trying to start up - Starter Motor ?

Clipper 1.8 Cabrio won't start without jump leads. Garage checked battery OK and alternator (original) was giving 13.8 v. Was only getting a click until I disconnected and reconnected all starter motor leads and now get the ratatatat (machine-gun) sound when key is turned. Car was also taken a good run just before and was charged up as was able to lower/raise cabrio roof. Always starts first time when hooked to other battery.

if it cranks over fine when jumped, your battery is duff

Could be a bad earth if when when you are connecting jump leads the negative is connected to the engine on your car as it should be.  Try checking and cleaning the main earth cable from the battery to the engine if this is the case and you are sure your battery is good.