Front door multi-point lock slowly getting harder to lock

Hi,

We've got a front door multi-point lock, of the "pull up to lock" variety. Multiple bars slide across up and down the door when this is done.

It is steadily getting harder to lock. Some real vertical force is needed. It seems this is a long existing problem which the previous owner neglected to fix, as the door handle itself has worked itself slightly loose due to the forces being exerted on it.

The door locks much more easily when it is open, which would point to an alignment/warping problem in the carpentry.

Any basic advice, other than liberal coatings of WD40 to begin with? ;)

(The combination of the new house, handle being loose, and locking problems mean we are more likely to call out a locksmith to fix all of this -- including new locks for the security aspect. What sort of price should I be looking at, when getting quotes?)

Thanks,

Tris.