After doing the head gasket i cant remember where this plug goes, it sits behind the tb

If that connector is single wire it must be the oil pressure sensor just next to the oil filter.

Unfortunately not, oil light works fine. It's at the back of the engine by the throttle body

can it reach the alternator?

It can but it seems to charge fine, the only thing i was thinking was a temp sensor as when it warms up it starts to run poorly

Temp sensor is a possibility. But I can't see anything obvious. Can't even be sure what colour the wire is -- red? pink?

All the connector types/wiring colours/etc are in the Seicento Technical Manual, free, gratis and fer nowt in the Downloads Library from this esteemed forum.

It's easy with hindsight, but a magic market and some masking tape labels takes moments.

Downloaded tech articles, looks like temp sensor, thankyou all for help. Great forum

Just in case you havnt done this already this plug is located at the back of the engine on right hand side