Do i have to get the butyl tape like this?

http://www.ebay.co.uk/itm/BUTYL-RUBBER- ... 1561165192

or can i use this?

http://www.idealsealants.com/non-silico ... GwodnN0IMQ

what bit of the headlight are you trying to seal? They are sealed units.

They are until you strip them

Ive taken a spare headlight apart and fitted a HID projector and an LED for the side light

I'm not sure I understand. I changed my bulbs for halogen but they just push in the back of the headlight unit. I never sealed them in or anything.

Are you talking about a 1290 light unit? Maybe they are different to my 990?