Had since start of August done 1000 miles in it, marked expansion tank when bought it, just had to top up by 90ml of coolant, I know it's not much, no sign of leaks.. Any ideas.. 

  TIA

Not so sure the expansion tank is an an accurate way to gauge, hence its min/max markings.

August was particularly good this year, I would imagine natural expansion/contraction due to external temperature may contribute to such a small change.

Thanks Chris, I'm thinking that just need reassuring 😂😂