Degraded brickwork and filler leading to gaps around window

You need a local window fitter or joiner to fill the gaps with sealant/mortar/expanding foam and then fit upvc trim to make it look neat and then re-seal... hope this helps

Obviously without seeing them it’s difficult to make an accurate judgement, A possible option might’ve to remove all the sealant and cleaning around the frame and then using a low expanding foam around the frame, then using plastic trim over the top and then seal down the plastic trim with clear silicone. The deterioration around the frame may need dealing with if it is severe or dangerous so this is only a suggestion but I’d be surprised if the deterioration of the brick work has happened within the lifetime of when that window was originally fitted 10 years ago. Good luck I hope it’s a cheap repair.

Would need to fill gaps in sandstone and shape mortar filler to the shape and size of existing stonework. Then use lignum paint same colour as stones and cast silica sand on top of wet paintwork to finish off

I would remove all old silicone and loose sandstone. Put some expanding foam in between stone and window. And then I would scribe a plastic trim to fit over the trimmed off foam. Then seal off trim.

You would need to use low modules sealant for this, it’s designed not to shrink