Just thought I would update this thread I started by confirming the exhaust valve on my car is still moving freely.
The car is nearly three years old now and just coming up to 8000 miles.
All I have done is reapply some WD40 high performance silicone lubricant around the valve shaft every 3 or 4 months as i originally suggested.
It certainly seems to me like the answer.

I think the exhaust valve when working freely as it should does increase the low end torque below 3000 rpm though.
My original rear exhaust section was replaced under warranty due to a seized valve and I noticed that the torque seemed to increase at lower revs.

Just another update on the solution I suggested back in April.
I have re applied the silicone lubricant just twice since my original note on this thread and the valve is still perfectly free.
So with just under 6000 miles on the clock now It seems to me a reliable solution to the problem if the lubricant is applied say 3 or 4 times a year.

My 124 spider has just passed its first birthday and has received quite a lot of under warranty maintenance work at the dealer. I mentioned to the dealer on its first service that the engine would misfire and hesitate if pushed hard and that there seemed to be lack of torque. I also stated that I was fairly certain that the exhaust valve had seized open. Whereas this seems to be desirable to some on this forum I wondered if it could be contributing problems.
The dealer confirmed the exhaust valve was seized open and replaced the exhaust rear section under warranty but the engine problem was referred for attendance by the FCA technical specialist at a later date. The FCA tech wizard replaced all four coil packs on the last appointment at the dealer and I can now say the car drives as it should with a much better throttle response and no misfires.
After an initial check of the new exhaust valve which I found to be a little tight and squeaking when moved, I decided to spend some time thinking about how to try and prevent the valve on the new exhaust system from seizing.
I chose a high performance silicone lubricant from WD40 (£5.40 at Halfords). This comes in a spray can with a folding directable spray pipe and is specified for use up to 200 degrees C which should be Ok at the tail end of the exhaust system.
During the first application to the upper and lower parts of the valve shaft, I worked it in by manually opening and closing the valve many times. With a few more applications and manually driving the valve, the operation is now very free and with no sound of dry metal on dry metal squeaking. I intend to make this a regular maintenance task every month or so as it only takes 5 minutes without even having to jack up the car.
I will try and think on to post an update later in the year and let anyone know who may be interested if the treatment is still working.