Are these purely mechanical or partly electric.
Earlier this week, mine stopped engaging when I removed the ignition key. Two days later the battery was flat (it wouldn't even turn the starter motor).
An overnight charge worked, as did the steering lock.
The battery is the original, so very likely needs replacing. But if a faulty steering lock is somehow draining the battery, a new battery will still suffer from the same problem.

 

just take the cowling off, put the key to number one position, by the lock theres a small hole poke a small screwdriver in and pull out the lock form the casing, clean it up should sort it

 

Thanks.
It was the fact that the steering lock didn't work when the battery was flat, but did after an overnight charge that made me think there might be a connection. If you're saying the lock is purely mechanical, I'll assume it was just a coincidence.