We have just had a new rangehood installed and there is a slight gap (approx 0.5 cm) between the top of the wall tiles and the bottom of the rangehood (see pictures).
I would appreciate some advice on the best filler to use here please.
Welcome to the Bunnings Workshop community @adamd. It's terrific to have you join us, and many thanks for your question.
Any standard silicone filler such as Parfix 300g White Bathroom And Kitchen Silicone would be suitable for filling that gap.
You should find this step-by-step guide useful: How To Silicone A Gap.
Please let me know if you need further help or had questions.