I went to my local pound store and bought the following.....
A Red glow stick (2 for £1)
A Push button Red LED torch (£1)
Black electrical tape (£1)
Firstly I took the led torch and took the plastic 'lens' off the top.
Next i got some sand paper and lightly sanded the glow stick, taking care not to snap it and make it glow.
Sanding it makes the light diffuse more.
Next i took a dremel and made the top of the torch bigger to fit the glow stick into it securely.
next i took the black electrical tape and carefully wrapped it around the torch, cutting around the push button.
finally i inserted the glow stick and glued it into place using a glue gun.
At the night of the party, i snapped the glow stick so that there is a constant glow, and whenever the push button is pressed it creates an even more powerful glow.
All together it took around 10 mins and cost £3.
Perfect addition to a darth vader costume :D