HLM Fluid

[Jez]

How often, time or mileage wise should this be topped up[?]

[alan s]

Depending on it's use and climatic conditions, about 3 years or 60,000 klms (35,000 miles) for the changes or if it starts to show signs of degradation (ie) yellowish colouring, moisture contamination etc.
As far as topping up goes, this shouldn't be necessary unless you have a leak in the system. There is no reason the car should "use" any.
Alan S