What do you do when someone locks their bike to your property

Someone has left a bike locked to the metal fence outside our house for a week without collecting it, it's really quite annoying!

Are there any legal complications I should be aware of if I break the lock and remove the bike?

A week isn't long enough.

First determine whether it's in use and let the owner know that it's not OK to lock against your property. Do this by putting a notice on the bike (A4 inside a plastic sleeve stapled to itself over the top tube is good... non-damaging and weather-proof but owner would remove it before use).

If after some stretch of time (1 month, say) it hasn't been moved (notice is still there), then consider it abandoned and take an angle-grinder to the lock and leave it unlocked for someone to remove (if it's there for more than 60 minutes you live in a very nice area).

It is absolutely NOT OK to act without warning or notice and to act too swiftly. If this were your bike that you were locking in a place that you thought safe think about how you'd react if you returned to find it cut-free, damaged or removed... just put a note on it and communicate it, and give it time for that to be received by the owner.