Before you spend out try to be sure that it is a lack of ventilation causing the mold. It can sometimes be walls made damp from outside etc. We had rainwater gathering in a faulty extractor fan once which was the problem.
Agree with all this, but with the caveats of a) think about it logically and b) if taking 'professional' advice consider whether they are just trying to sell you something.
Still remember a bloke coming to me wanting to install a DPC into a listed building as he had damp at the top of his ground floor walls and one of those DPC companies had come out and told him he had rising damp and needed a DPC having checked it with one of those moisture meters*.
When I queried with him (and then at his request the DPC company) why the damp was apparently coming from the foot of the wall but undetectably going up the wall and only manifesting at the tops they were unable to explain how it was migrating up the wall, once again it was a condensation job due to solid walls, no cavity and poor ventilation.
Loft moisture is a real problem but no surprise when you think how cold it gets around the felt/tiles v heat seeping up through the ceiling even though there's insulation above. What you've done sounds a good idea and clearly works but you would think there would be enough air flow from the soffit area to prevent any condensation.
Hence why breathable (and ridiculously thin compared to old fashioned types) felt is pretty universally specified on jobs these days.