Should I be checking anything or just topping up the fluid?

As a normally sealed system, the only 2 ways for the fluid to be low is if you either have a leak or the pads are very worn, requiring more fluid in the caliper pistons. I would initially check the condition of the pads and whilst you have the wheels off you can look for leaks on any of the joints (wet looking). If the pads are well worn, replacing them will force the pistons back and push fluid back into the reservoir.

Many thanks. I'll investigate!