I've read in numerous places that you can polish polyurethane varnish. I have an area on my foredeck which i've tried to polish with varying combinations of:
1200 grit wet paper
brasso
metal polish
rubbing compound
car polish
buffer attachment on drill

Nothing seems to come close to the original finish (less the dust and holidays). 

Any advice greatly appreciated, I want to get on with the fun part of fitting out and trying to get the boat sailed before its too cold!

You need a proper electric polisher, a drill attachment won't cut it.

you may get better results using 1500 or even 2000 grade paper, but without the correct tools you probably won't get bthe result you want.

Just sand it down again and put a proper coat on top in good conditions, draft free, wet floor, surface wiped down with a tack rag and a clean but well used brush so it will not drop hairs. It will quicker in the end. 
Varnish will apply better if the varnish is slightly warm and harden better if you do it early in the morning in a rising temperature gradient.

I assumed he was talking about two pack varnish.

One pack polyurethane should not be polished until at least a week after application. It needs to go thouroghly hard first. With two pack in the right conditions you can do it the next day, though 48 hours is safer.

The "warm varnish" trick i've found causes problems with one and two pack polyurethanes, but works well with stuff like International's Origional.

Flatting and polishing is quicker than doing it again provided you have the correct tools and don't go through the varnish! Some surface dust is almost impossible to prevent even with thorough precautions.

At work (small chandlery in Gloucestershire) we use 2000 grip wet and dry, use water mixed with 3M rubbing compound and work at it, then polish with massive makitah buffer with rubbing computnd.

OO yea, and the key ingredient-ELBOW GREASE!!

Gents, thank you for the advice.

After this 6th coat on the deck, the inside and the outside of the hull both filled faired and painted with epoxy and 2 pack, the elbow grease is running low, along with patience. 

I sanded and re-varnished. It looks OK, but not perfect. PU varnish is a nightmare to work with. Cant really extend the budget to a polisher at this stage, with entire boat to fit out; 10 blocks just for the shrouds, sails last week, etc, etc.

I will pursue 1500/2000 grit and 3M rubbing compound over the winter I think, but I want to go sailing before its too cold!

Thanks again

One of the tips passed to me was to be rigorous about working through each grade of abrasive – it’ll take a long time to get rid of 120 grit scratches with 1500 grit wet and dry,  so be patient and go finer in steps, not big jumps.

I got a great polisher from e bay which wasn’t expensive, £39.00 + p&p, works very well indeed, and comes with a few sponge mops.  Definitely not a lightweight Halfords job for buffing wax off a a bonnet.  A genuine bargain.  It does spread rubbing compound all over the garage in seconds at full speed though! Only needs a light touch – let the abrasive do the work.  The guy has sold a lot of them.

Link below - Handy even if you do it over the winter.