I recently brought 18 Mains Halogen Potlights from B&Q, and found
after fitting that 6 of the halogen bulbs failed within 3 days.

How did you install the light fittings? Perhaps the biggest effect on
halogen bulb life is the method of installation. In particular, these
fittings run hot and need good airflow to keep the temperature down. If this
doesn't happen because they are fitted in an insulated roof void, then the
bulbs will pop like corn.

Christian.