I'm a novice, so I hope you can help.
If I write a piano score with a bass and treble line and save it as one midi file then import it into HS7xl, it opens up 2 midi tracks for say, TTS 1 but any voice I try to set it to, always reverts to Piano 001.
Yet, if I import the Bass and Treble score as two individual files I can point either at TTS 1 and the selected voice works fine. I realise the problem is something to do with assigning the tracks. Is there any way around this? Help!

Use a different midi channel for each part. 
 
TTS-1 has 16 channels , the default is piano for all but ch 10 which is the drum channel. 
You can change each channels instrument by clicking on the GUI beside the fader. Of right clicking in track view and use preferences to select a bank and instrument. 
Change channels there as well. 

I've done that but each time it gets to the midi part it changes back to Piano 001. If I loop the part and change the voice while playing it changes but both tracks revert back to Piano 001 as it loops.

Try opening Event List View.  It may reveal something relevant.
 

The Event list doesn't seem to show anything changing. I just tried pointing the track to
another synth which plays an intro etc and sure enough when it got to the treble and bass
stave parts it reverted to Piano 001.
I think the problem must be that when I'm writing on the staff the two clefs are "joined" but
HS7 splitting the track leaves some info behind? Does that sound possible? Although I'm
a novice I don't recall this happening before.

You might be a victim of Sonars MIDI PG is hidden from you. It should show in the event list but they for some reason do not. I mostly run into your issue with downloaded midi files where the author added a PG change mid song and thats what happens. I do not understand why Sonar is the only daw I have used that hides the PG data from you.
My only solution is to open the midi file in Cubase first, and delete the PG message, save and then open in Sonar.  
Use any midi sequencing program and go look in their event list. There are a lot of freeware simple midi sequencers if you don't have another daw on hand. 

Just an update, I've just tried it with a completely different VST (Dimension LE) and it worked perfectly. So I'm guessing either TTS doesn't like what I'm doing or it is somehow corrupted.

There are huge differences between TTS-1 and DimLE. DimLE only handles a small subset of MIDI instructions where TTS-1 is a GM synth designed to handle a wider variety of MIDI instructions. Usually TTS-1 is better at handling MIDI generated outside of SONAR but this appears to be an exception. It is not clear what tool is being used to generate the MIDI data but many have options to control the data exported to the MIDI file.

Bump to remove spammer topic title from home page.

Flashart
Just an update, I've just tried it with a completely different VST (Dimension LE) and it worked perfectly. So I'm guessing either TTS doesn't like what I'm doing or it is somehow corrupted.

TTS-1 is not corrupted. It is following an explicit command embedded in the file.
I think you are very close with this is statement:
 
 "I think the problem must be that when I'm writing on the staff the two clefs are "joined" but
HS7 splitting the track leaves some info behind? Does that sound possible?" 
 
Yes, I believe it does. When you import this file, it contains information that tells Sonar to create two MIDI tracks, and specifies bank and patch settings for both. When you play this file, Sonar responds to the bank/patch data (the program changes Cactus mentioned) and TTS-1 simply responds to the instructions it receives.
It would appear that when you split the MIDI into two separate files and import them, bank/patch data is being stripped. This leaves TTS-1 free to respond to internal settings, or those from the track settings in Sonar.
There is a hierarchy at play here.
1. Embedded program changes(bank/patch) will override everything.
2.Bank/patch settings in your track pane will override selections in the TTS-1 GUI.
3. If nothing is selected at a higher level, patch settings in the TTS-1 GUI may stay put.
 
Since you have discovered that importing the MIDI tracks as separate files is more cooperative, I would be inclined to move forward with that method.
Otherwise, if possible, you may be able to open the event list in the source program, and remove bank/patch instructions there.
 
As scook alluded to, DimLE does not respond to GM bank/patch settings. They mean nothing to it, so it ignores them. On the other hand, TTS-1 is a dedicated GM2 synth, and it responds to all GM instructions sent to it, intended or otherwise.
Sorry for being so long winded, but I hope that helps.