can anyone tell me what the black cap is and possibly why its leaking oil please

Any advice appreciated

thats the oil filter cap.
replace the oring on it. comes with oil filters usually.

could also have a hairline crack from being overtightened

Yeah same as above replace the o ring or it could even be loose? Get a 32mm socket or adjustable spanner and check.

Ta have tightened but guessing o ring will sort monday