What sort of finish are you looking for here? The easiest option might be to take a length of 90 x 45mm outdoor framing timber and trim it up to fit tightly within the overhang and gap. It can then be painted to match the surrounding area. Another idea could be to take a length of decking and fix it to the face of the overhang covering the gap whilst also making a feature of the step.
If you are handy with a trowel, you could potentially fill the majority of the void with expanding foam and then apply a Lanko 5kg 136 Rapid Patching Mortar coating over it. You can then use concrete paint to match the porches colour.
Please let me know if you have any questions.
Hi @Fitzhouse , I agree with @MitchellMc's ideas, with one possible variation... Is it possible to remove the bottom board and move it up against the top board? If so, that would move the gap to the bottom. Then you could do MitchelMc' s patching mortar suggestion at the bottom where it joins the bricks and it would blend in better.
As @MitchellMc suggested, I would turn this into a feature step by placing decking over it, it's the perfect height for it.