So I have this part which has been modelled using surfaces and solid features. I've been stuck trying to create this feature: widening the hole shown to 14.5mm. It cannot complete because of "invalid model geometry". I've tried saving the part as a .step, importing, healing faces (initially there were 324 invalid edge/faces). I did this a few times until I was left with 2 invalid faces. I then removed the invalid faces from the geometry with an extrude cut.

Alas, even though there is no invalid geometry remaining, I still cannot create this simple cut extrude, resulting in the same error message: "Operation failed due to geometric condition".

I'm at a loss on what to do now, does anyone have any advice? I've attached the part with the sketch in that I'm trying to extrude cut.

Thanks for your help in advance.