Im trying to make a carbon fibre bonnet but am stuck with the matting not being wide enough.. is there a special technique to lay is so when you pull it out of the mould there is no visible join on the surface?

If you are overlapping 2 sheets of carbon in a mould for resin infusion (or similar) in say a "V weave" pattern, then you should not get a physical bump or line once complete on the moulded side.  You will get a slight hieght difference on the reverse peel ply side.

To get the weave to seamlessly go over a overlap join would be very very hard to do perfectly. It would be easier to make a feature out of the join.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

A few years ago I had the honour to visit a composite shop and was allowed to work for them for a few days. They had a lot of interesting projects going on, among them a prototype bonnet for a Mercedes Benz car still in development (taking pictures wasn't allowed, unfortunately). I helped with cutting the carbon fiber and laying it up in the cnc-machined mold which was then infused with resin. The carbon wasn't wide enough (this bonnet was pretty large, apparently wider than 1.5 meters) so they decided to use two sheets joined in the middle. Looks better anyway when the lines from the twill weave kind of point forward and meet in the middle. The fabric they used was a very heavy custom-made weave (at least 600g/m², probably more like 1kg/m², looks awesome on larger parts!), and we simply cut a strip of lightweight 200g or less plain weave fabric and placed it on top of where the two carbon fiber sheets met. Kind of covering it up with a straight lightweight strip of carbon fabric about 1 inch wide, placed in the mold first before laying up the rest.
I never saw the finished piece since I left before demoulding, but they told me they had done this before and it usually looks pretty good when done right (straight and no fraying).