1: Will I need to change my current spark plugs ( NGK Iridium 3667 BCPR6EIX-11 ) and if so what type should I use?

don't know those plugs at all. i use ford 071's on basically the same set up as you that are just a couple of quid each. i would love to give you a part number, but the search function is not working

2: Can I put a recirculating Dump Valve ie. Bailey DV30 on the coldside and run the release hose all the way back across to my airbox?

yes. that's a good idea, better than the original place with the hotside air being recirculated by the d/v

3: Is there any way I can silence my current DV24 Dump Valve?

a non recirc d/v can always be wrapped in foam to help silence it. or you can put it on a long hose and into the airbox effectively turning it into a recirc one. it may not dump as much as it would when mounted directly on the cold side hose though, as there will be a pressure drop along the long hose. i run without a d/v and have no problems with the turbo and find the engine more responsive on gear changes

4:Is there any danger in swapping between using 98 Octance and 104 Octane fuels in my Cosworth?

is that 98 and 104 RON? if so, 98 is plenty. 104 will not give you any more power on astd chip as the ignition advance will not be mapped in to take advantage of it, bt it will give you a saftey margin against detonation if you have any kind of sensor problem. depending on the cost and how much mileage you'll do, you have to chose which to run.

5:What is a safe boost to run with all these modifications done (bearing in mind my turbo hasnt been rebuilt, its 17 years old) ?

[b]depends what the chip is mapped to, normally about 22 psi held. i run that with no problems on my std 2wd t3. check with the chip supplier/mapper though. ad make sure that the map is for your turbo spec, not for a t34. a t34 flows more air at the same boost pressre, so a t34 map will be rich on a t3 engine[b/]

6: Is there anyway to safely adjust the boost without having a "Boost Gauge" installed?

you need a boost gauge of some sort installed, but not necessarily a manometer type with an air connection. the car already has a MAP sensor, so you can read the output of that sensor on a voltmeter and convert it to pressure.

7: Is there any tuning that is required after installing a new chip or is everything programmed into the chip and you just drive away?

theoretically no, if the map is suited to your engine spec. a setup should be done though, but that's something different to tuning. you should have the fuelling checked with a proper wideband lambda sensor under all operating conditions. the plastic screw on the ecu can be adjusted to alter the fuelling over the entire rev range, but you should know what you're doing before fiddling with it. the fuel pressure should also be checked. also, a fuelling check at idle does not gaurantee an adequate fuel supply at high rpm+boost. if your chip was bought from a reliable supplier, it should be good and have a relatively high safety margin built in as the spec that you will be running is well known

8:

i can't answer no. 8