My cordelines leaves are going brown from the tips - how do i stop this?

The leave's die off Naturally Sam, Once theyv'e turned Brown just cut them off, and you will see a Trunk, Cordylines are Trees :)

Could be a combination of factors here Sam. As Youngdaisydee says, the older, outer leaves lower down on the stems do die back. In wild plants in it's native New Zealand, these dead leaves serve a function forming large "grass skirts" around the trunks helping to insulate them from the cold they sometimes experience in their mountain home. They look aesthetically better to us of course removed and most people cut them off back to the main stem to make the plant look tidier.

Are they the normal green form or one of the less hardy coloured variants?

They may be slightly starved and root bound too of course.

One other question, in pots you must adhere as all plants to their particular watering requirements irrespective of any rain. A good soak (say 1-2 full watering cans 2gallon) once a week letting dry off a little between is usually best for them. If they stay too wet at the roots, even the young leaves can turn brown.

One last point, some of the browning may also be due to winter damage and is only really starting to show now.

i have noticed in norfolk but its possible the hole country how many people put these trees in a small pot.the amount round here just going yellow and obviously not happy.another amazingly commen mistake is people buying plants,watering them in then hoping the rain we get is enough to keep them alive.mostly missinformation i guess on unscrupleas excuse my spelling sales people who want to sell you more plants and have no real passion for it.