Halfords do an 063 but its 12Kgs, their 053 is 13Kgs
But I found
this clicky which claims 9KGs,
my car currently (no pun intended!) has an 075 which is 15KGs so thats a 6KG saving and my other battery will weigh in for a tenner at the scrappy.
I will look into a M/C battery as I'm happy to use a booster pack for startup cranking as its a track car.