I'm wanting to add a few degrees of castor to make the steering less nervous. The mx5 lower ball joint bolts into the non adjustable wishbone as shown below. I was thinking about getting 10mm or so machined off the forward face of the ball joint housing and and adding an equivalent spacer to the rear face to bring the ball joint forward. Any reason why I shouldn't do this?

Cant see why not, are the wishbones to chassis sphericals or bushes

I may be talking out of my ass, but you may be altering the track rod end angle the same as you would if you lowered the steering rack - if this is true than you may be introducing some bump steer?

It's not that simple. See the middle bolt in the wishbone? That's also part of the ball joint. You'll have to think of some way to move that bolt sideways. Elongating the hole won't work, it'll fail the MOT. Off the top of my head, welding up the hole and drilling a new one might be ok? But the MOT bloke will still see the balljoint has been modified.

Won't the cam bolts on the wishbones give you enough adjustment to change the castor?

Forgot to state the obvious in my post that its on my mx5 based zero not an actual mx5. So whether the wishbone is modified or not need not bother the mot man (providing its safe). And yes I was just going to slot the hole for the vertical bolt.