Hi Steve,
I haven't heard of anyone in the U.K. repairing HP41s, but Steve Kersey in the US does, and I will give it a go if you want, I am in Australia.
If you are willing to open the calc up, try squirting some circuit board cleaner into the hole behind the switch, then exercising the switch for a while. Get a circuit board cleaner that doesn't leave any residue. I use the "Ultrasolve" product. Opening the calc will also let you see if you have a cracked plastic problem. Both worse case scenarios, broken plastic screw posts, and the switch need replacing are both possible to rectify, but they are both time consuming.
Cheers - John