We have existing polished timber floors, it is patchy (different parts of the house are floored with different kinds of timber) it also has a few gaps and holes. We are thinking of laying down some kind of timber veneer over the top, is this a good idea and if so what are the best materials available? (Ideally it would look as authentic as possible)
Engineered hardwood flooring is what you're looking for. It's a thin layer of real wood bonded to a backer and pad. They snap together tounge-and-groove style. You may be able to install it directly over your existing floor, but you'll have to check with the manufacturer of the particular product to see if that is an allowable installation method.
I have done work with this company before:
..they reclaim old barn timbers and turn a lot of them in to flooring. It is usually 100-200 year old Douglas Fir in our area (upper mid-west) and it is authentic and looks old and beautiful.
My suggestion would be to find a company like that in your area.
A good sanding and refinishing could solve your problem. There are all sorts of tinted fillers that can be used to fill the gaps and holes. If the floor is relatively level and solid, than this is probably the way to go.
I personally do not recommend this as a DIY project. It can be done, but it requires some hand skills and experience that a professional will have and you will not. They also likely have a half truckload of fillers and tints.
It will take about 5 days to do the job properly. (1 day sanding, + 4 coats varnish)