It's also incorrect that the 'sustain' pedal doesn't have more than an on-off function in reality, so the Garageband missing manual is indeed full of brown stuff! :-)

Regardless of the pedal issue, the recording is very good. Mind my asking which preset you used?

Thank you, Jake. I used the YC3 Chamber detuned with the reverb and limiter switched off. I may have hardened the hammers at piano a touch (they're at 0.37) and I used the reverb on Garageband at 14%. That was actually a mistake; I'd meant to use the pianoverb plug in, but forgot to switch it on!
Regarding pedalling, I do use the sostenuto pedal for the long held bass notes when the score goes into 3 staves, but use half pedaling shortly after to hold down a fifth in the bass across some conflicting harmonies; so it does work!

Well, I'm embarrassed to report that, after rereading the quote from the missing manual, and then experimenting, I find that although garageband shows the sustain pedal as either on or off, it does record a full range of values for it. Odd.

Hehe... I wondered why they should expend effort to change the incoming data to either 0 or 127... Well, they don't. But funny they do it for the display, although I understand it for notation purposes (the "ped" and the"*" sign).

"After all, in the real world, there’s no such thing as half-pressing the pedal on a piano."

So much for the GarageBand maker's real world.
Maybe you can use Controller No 4 "Foot controller". Refer to the MIDI Standard Controller Numbers.
Of course your pedal must be able to send using this number and Pianoteq must be able to use it (which I don't know for now).

Maybe there is one solution if you can configure your pedal to send a controller number different from 64 (Hold Pedal) and use a number that is designated for continuous values and make Pianoteq take this controller number for the sustain pedal. I think in former MIDI times for the hold pedal any value other than 0 just meaned "pressed", and although it is no effort to record continuous values, GarageBand seems to decide what is useful... In its own way.

Yes, I thought that the answer might lie in that direction; I can't find any information on what controller number Garageband takes for its expression pedal, so I guess some experimentation is in order.

In the meantime, according to "Garageband, the missing manual" I don't have a problem: "As with key velocity, sustain-pedal motion is recorded on a scale of 0 to 127. Unlike key velocity, however, there’s no such thing as a sustain-pedal setting between 0 and 127. After all, in the real world, there’s no such thing as half-pressing the pedal on a piano."