This has somehow happened today 

Any ideas on the best way to repair?

Im wondering whether I can heat it up (hairdryer?) and bend it back to shape?

Or whether I need to cut / file some of the bent bit off to stop it getting worse?

Or is it pointless and new strip time?

New strip might not do any good either. Had the same problem on mine. The cause is the door sagging. (worn hinges) Bought a new strip and about a month later same problem. Looks like you have the same problem from your pic. Gap between door and fender seems smaller on the botton than the top. I trimmed my strip down to size with an anglegrinder and glued it back on.