A photo will be very useful :-)

Bear in mind that carbon doesn't like being bent anywhere near as much as Aramid or fibreglass.

Thanks for all of the responses guys appreciate it. I might not have explained the requirement too well and I'm still a bit early in the plug making phase to be able to post a photo that will help withbthe explanation. The seat is a basic pan shape, maybe 10 x 8 inches with two forked prongs on either side that will bolt to the frame in the front. It has to withstand the forces of let's say a 120kg rider sitting on it acting as a lever on the front four bolts, plus the additional forces created through suspension travel on a normal road. 
I've looked at a similar design today on another bike and the carbon seat pan is about 6mm thick so maybe my 10mm estimate was a bit on the high side! 

If I aim instead for 6mm, I presume I can do this as one single infusion of several layers of cloth, would you agree?

I understand what you are saying about steel plate Steve, but a steel alternative to the shape I need will weigh a lot more than the carbon version plus will add complexity with the seat cover and other items in the design so steel really isn't an option for me.

I'll see if I can take a photo of the plug so far that will illustrate better what I'm aiming for...

My rough and ready calcs indicate that a 10mm composite (40 layers of a 200gm cloth) will weigh around the same as a 1.5mm steel plate.

How do you plan to mount the seat? If by springs or direct to the frame, then the metal seat will be easy to weld. Not sure how you would mount a composite seat without extra brackets etc.

10-12 mm solid laminate are definitely overkill for this application. This would be so heavy that a steel subframe should be lighter. And incredible expensive btw.
Your seat should be easy closed structure, I guess. if so, way less wall thickness should be sufficient. I can't give you a actual number, but it should be something around 2-3mm at the base, and less and less up to the tip.
And if you really have an open structure, the main problem should be the bending and torsion forces. And for this cases, a sandwich construction would be advantageous. You can infuse sandwich parts with a soric core.
If you aren't building industrial parts like wind turbine blades, over 10mm solid laminate would never be necessary. Even over 5mm.

I've made a few parts for my motorcycle project using vacuum infusion and the products available from easy composites and am moving on to the seat now. I need to make the seat pan between 10 and 12 mm thick so it is self supporting, I.e. no metal subframe. The items I've made so far probably peak at 4mm thick. Can I make a 10mm thick part in a single infusion or is the technique to get to 10-12 mm thick achieved by running successive infusions of smaller thickness? Grateful for any experience people might have on making thicker parts.
Thanks 😉