I would like to cover the Kitchen Worktop, in my camper with a single layer of carbon cloth and hi temp laminating reason.

I was intending to use

Carbon Fibre 2/2 Twill 3k 199g

Very High Temperature Epoxy Laminating Resin 500g Pack

I have done a few structural repairs with carbon before but that is it, so I understand how to lay the fibre and resin, but I am looking for advice or a guide on how to get a flat gloss finish, ideally a mirror like one.

You're going to have to do some flatting down and polishing. You'll never achieve a perfectly flat surface through application alone. I would recommend getting an orbital sander with some fine sanding discs, say up to 800. Get it flat with 400 first, then move up to 600 and then 800. Then continue with wet sanding by hand up to 1200 or 2000 grit, then compound. 

the worktop is 0.5m2, but nearly half of the space is occupied by the stove/sink, so if I get enough resin for 0.5m2 that should be plenty. How much area will 500ml cover?

Ah 500g will be heaps then. With wet-lay and one layer of 200gsm carbon, you would be aiming for a 50/50 ratio, ie. 1g resin for every 1g fabric. So at 0.5sqm, that would be 100g of resin. 

Do it this way:

key the existing top with 180 grit (orbital sander is fine)

add black dye of some sort to a small batch of  high temp resin, then put down a thin coat of that on the worktop ..
Leave it to harden to a weak tack ... then lay down the CF cloth, and laminate with an almost excessive coat of the resin.

Leave that to cure to a tack, then lay down at least one more layer of resin.

Then leave the whole thing to cure fully ... Then you can start sanding it flat and polish it...
Be aware that it will never take on a real "mirror" shine ... Epoxy just does not polish that shiny ...

Good luck  

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Would you recommend that I put anything over the resin, glass, plastic? Or just leave to cure?

thanks for the advice so far!

I would get a piece of glass , suitably waxed and PVA released and stick that on the top, and weigh it down a bit, should give you a much nicer surface finish

Would acrylic, with release wax do the job? Easy to get hold off and maybe to release...

as long as its suitably released it should be ok, I would weigh it down with something too

Need a bit of help/advice on how to proceed...

Laid the carbon down with the excess resin, the carbon is down nicely etc. But as I ran out of time... I rushed the last coat of resin, and I could not get hold of polypropylene or glass sheet to put over it.

anyway...

I was thinking of flatting it back using my sander 400 through to 2000 and polish etc as recommend in this thread.

But I would ignore the worst of the low spots, sort of like a practise run...

Once I done my best put another thin coat of resin to fill the low spots and sand and polish again. That way I get a bit of practise... so when i do my final coat I have more idea of what I am doing...

Does this seem like a workable plan? what will I need to remove the polish compound if go down this route before I overcoat with resin?

I could get a polypropylene sheet, and do another layer of resin, though I am loathe to do this now, as I still may make a mistake. At least if I flat off and polish what I have now I will have a practise before I do the final coat.