Is it possible to add further layers of prepreg to a cured part and re-bag & bake?

Will the resins bond ok to the cured parts - I'm assuming the cured part will need keying up?

Thanks in advance

Yes.  It is just like with wet-layup.  You will need to prepare the surface for secondary bonding by sanding and/or peel ply removal - whatever it is you do.  Most times people will use a resin film/glue film/film adhesive to add a little extra (often toughened) resin to the bondline.  If you are using high resin content pre-pregs or many layers (more resin) it may be fine.  I'd suggest either heat-gunning or a warm debulk to make sure the resin on the first ply down soaks into the cured carbon and there isn't trapped air.

Also be very wary of how clean your mould surface is. Any debris on the mould surface, no matter how tiny, will punch a dent into the component when you rebag and cure.

Cheers guys good know it will work, I've found the easycomposites prepreg very good with quite a rich resin content -  will take also care with vac bagging I've had my share of frustrations with maintaining a vacuum because of an unknown pin pick somewhere!

X-Preg isn't resin rich, it still only has a 42% resin content which is pretty standard for pre-pregs. If you're only adding a layer or two, I would still recommend using a layer of adhesive film to get a good bond to the surface.

I do this all the time without film glue. Works great.