Hi all,
Welcome to the forum!

Just joined the form after seeing that clip from fifth gear on the Polo. I was amazed by the difference!.
Nitrous can make all the difference in performance, especially on a diesel!

I'm new to car modifications, although I like to dabble in a lot of different technical ventures.
Nitrous is actually a great place to start because of it's simplicity.
Simply jet up or down to change your desired power level. Since you're a diesel,
this will depend on how much fuel your mapper will be able to squeeze out of the stock fuel system.

Just want to ask the guys here who will know a lot more about the subject that me.
You've come to the one and only place on the web for that advice so good job on your part.

I going to get the car remapped soon, how will that affect me in trying to n2o the car?
That depends on the capabilities of your mapper. I'm going to assume that you're mapping for more boost, correct? If that's the case, then your turbo is only going to be able to go so far before it can no longer supply the airflow to burn the rest of the fuel the stock system is capable of. The rest of the fuel will need the addition of nitrous in order to make the extra power. Therefore, you may need to enquire about the ability to switch maps between turbo only and nitrous use because the nitrous map will be sooty and lower your fuel economy. You only want the extra fuel when the nitrous is to be used.

Will my clutch begin to slip with the extra power?
This is a possibility, but it depends greatly on how much power you ultimately make. With a diesel, it's almost impossible to predict the amount of excess fuel the car will have to match with nitrous. The only thing you can do is give the car all it's got and keep jetting the nitrous up to match until there is A)
no visible soot and B) no further power is gained from additional nitrous.

How much of a nos shot can I use before I am likely to damage my engine?
Being a diesel, you have absolutely 0% chance of breaking your engine from poor air fuel ratio as petrol engines do. The only way to break a diesel is as follows: 1) the mapper changes the fuel timing (a big no-no as results can be unpredicable when nitrous is added) 2) exceeding the amount of torque the engine is capable of (diesels are VERY strong by design so that usually isn't a problem)
3) the addition (and incorrect proportions) of other fuels introduced in the intake manifold such as propane,
alcohol, or other fuel. This is due to the precise timing and compression required for the diesel to operate.

And finally, how could I go about adding more fuel to the mixture (more diesel or methanol injection perhaps) to make more power?
You must add the excess diesel via the stock fuel system as it requires precise timing for it to burn in the engine safely. You can add methanol or propane along with water to add greater amounts of power. However, be warned that your mixture must be as perfect as possible or else you may cause damage.
Adding alternative fuels is not for the novice and I'd advise against it in the first years of learning about diesel nitrous tuning and your car's unique attributes.

Once you've gained the required knowledge to add the extra fuel, then the sky is the limit.

Thanks guys
I look forward to you're response

Thanks guys for your response!!
I've had the car remapped already and its making some incredible power compared to the stock map. But its still not enough...

Pardon me, It was a golf!

I'm looking forward to this project and it looks like I'll be getting my self a nitrous controller to give my clutch a chance. I've been struggling locating a more hardy stage 2/3 clutch that would fit my bravo 2. I've heard that the Alfa 14x parts are similar but I wouldn't like to gamble taking my transmission apart to find that it's not going to fit.

What exactly about the water methanol mixture needs to be so precise? Nitrous freezing the water perhaps? XD
Methanol doesn't need water until the nitrous levels are increased, but by how much is impossible to guess. If you use propane, it's generally a good idea to throw in 50% of water to prevent any preignition. I'm an advocate for propane because it's much cheaper and easier than methanol. If your increase on propane is small enough, then water won't be necessary. Again,
it's impossible to guess so adding water as a general rule is simply a safety measure.

I've also been looking for more beefy injectors or even just bigger nozzles with no avail
Alternative fuel sounds like your only choice here.

I'm saving my pennys now anyways so I can get the kit and controller, the remap skinned me £360!!

Thanks again!!

Let's just say I was going to be completely mental and decide to add water methanol injection into the mix later on, would the WON controller be able to control the water/meth delivery system as well?
Yes, it can trigger the water activation. You cannot do anything to control it progressively without a Pulsoid, however. Generally, it's not a big deal to do so anyway.

I don't think my car would be capable of switching maps on the fly, The most advanced thing on it is the trip computer. XD
That's something you'll have to work out or simply use alternative fuels. Remember, once you go this route then you will run the same risks as petrol engines in regards to detonation prevention.

I had it remapped my Celtic tuning because they had the biggest power increase quote. I didn't believe for one second that I would gain 65bhp from the remap but the guy assured me that it was legit. Might take it to a dyno to find out..

I'm in the U.S. so I don't know much about any of the tuning companies there, but it wouldn't hurt to verify their claims.