Thanks for the replies.

The battery seems fine as it's taking it's charge from the Optimate and changes from charge to maintain as expected.

The plug is the right one, gapped correctly and the colour is in line with what it should be - light greyish. I've ordered another one just in case I have a dodgy one.

I can't see it being the starter motor due to the sequence of when it starts and then it starts fine after warming up.