Looking at this from a layman's viewpoint:

If you achieve optimum cloth to resin ratio, adding graphene will surely reduce this ratio thereby weakening the composite as it takes up space normally filled by resin?

Even if the cloth can be thoroughly soaked, how does a graphene powder add strength as it isn't interlocking, just bulking up the resin?

Adding graphene will increase the composite's weight as it doesn't replace resin.

i am prepared to be educated as I love learning stuff :-)

It's all part of the matrix (and no, you havent taken the blue pill). The theory is that the fillers are in the entire matrix, handling and transferring loads the same as a fibre reinforcement is. Anything that improves the strength of the matrix for a certain application is beneficial - resin ratios are relative. A straight carbon/epoxy matrix may be at its strongest at a specific ratio to each other. But if you are adding another type of reinforcement to the matrix then there's no saying that the addition of that reinforcement doesn't allow that specific ratio of carbon:epoxy to change without hurting the mechanical performance, and in fact it may even improve it.

Realistically, it's quite complicated. As Oekmont said, the resin part of the matrix doesnt account for terribly much of the mechanical strength of a well made composite laminate. Certain fillers do slightly improve the mechanical performance in terms of tensile strength, compressive strength, yield strength, etc. But in a lot of cases, you improve one and hurt another. You can also change certain properties that the resin is associated with; such as conductivity, lubricity, density, shrinkage, thixotropy, etc. So generally with fillers you know what properties it can add, you know what properties it can take away, and you use the ones that give your laminate the properties that you desire. 

The problem with nanotubes and graphene and other additives like these is that they are SUPER expensive, and their benefits in composite applications aren't really a direct correlation to that price, so why use them? They may improve things a tiny bit, and if they were super cheap then people might use them. But they are just too expensive to be useful.

There is more to think about than just the price. What quality of graphene are you able to produce? What chip diameter, what thickness? Are you able to functionalise the surface to get a good bond to the epoxy resin? To get good results all this should be taken into consideration.
You cannot substitute the carbon fibres by adding graphene powder to the resin. There is something called critical fibre length. It is the length a reinforcing fibre should have at least to transfer all of it's strength propertys through the composite structure. it depends on the stiffness of both reinforcement and matrix, the diameter of the reinforcement, the adhesion strength, the form of the reinforcing particles (fibres are optimal, flakes are the second choice) and some othe factors. For carbon fibre we are talking about millimetres. The rule of thumb is 50mm. Making the fibre longer doesn't increase it's performance. While regular carbon fibres are not as strong as graphene, they are much, much longer (graphene flakes usually have diameters of several nanometres up to a micrometre). And they are fibres, not flakes. So although it's better individual characteristics, graphene doesn't increase the global mechanical propertys of a composite structure in a way a carbon cloth would do.
To me 25% seems much to much. You need the resin to glue the fibres together. So more graphene filler doesn't mean better characteristics.
In my opinion 1% and less graphene addition is the optimum for manufacturers. Because than you can add the word graphene to your pr and may sell some more products.

I used the graphene enhanced prepreg in the structural parts in our stiski project, we found that it allowed the T700 spec material to be as good as the T1000 high grade carbon materials we had originally designed too. Cost wasn't that much more expensive as its only a small amount contained in the resin system.

http://shdcomposites.com/case-studies/sit-ski-project/

If you are looking to make some money from producing Graphene then I would investigate speaking to some of the other prepreg manufacturers and resin system producers in the UK.

The graphene I produce has flake dimensions ranging from 0.25 to 50 micron but
to make 50mm flakes would require an altogether different approach to manufacturing.
That would need to be a bottom up method rather than a top down
one.and the last I checked, 1x sheet of 50x50mm single layer graphene is going to
cost somewhere in the region of about £500.

So for now, I'd have to conclude that graphene in powder form may not have the ability to improve
carbon fiber manufacturing enough to make it worth while but then this was not the sole composite
that sprung to mind either, there are others, particularly composites requiring high temperatures
such as ceramics.

As for what I can do right now, is to use it in thermal compounds and electronics.