I have just wired my boost gauge and installed in the a pod, but worried about what pipe to 'T' into under the bonnet??

Is it the one from the bailey dump valve to the manifold,

or the pipe from the turbo to the daws devise??

(its a 98' Classic 2000 turbo)

Prob a really simple question, but just wanted to make sure!

Mate the T piece should fit on the vacuum pipe that you remover from the OE DV, then onto you Bailey DV and the top bit off to your boost gauge.

Hope it helps.

There's a blanked off nipple on classics to tap a boost gauge in, try:
http://www.ostadal.pwp.blueyonder.co.uk/car_mods_gauges.htm

As Richie says, there is a spare nipple that seems like it was made for this. It couldn't be easier. http://www.ostadal.pwp.blueyonder.co.uk/images/cars/mods/gauge/step3_g.jpg