Yes Mike, as stated you should have another little knob under the dash, close to the clock adjuster, to reset the trip. Have a good look under there!
If it is not there then you can replace it, but the cable assembly is not available new & very hard to find in good used condition. You could try one of the TR7 specialists like Robsport or S&S.
Attaching it to the back of the speedo is a bit of a fiddly job too, you might be able to do it by reaching up from under the dash, failing that you would need to take the dash apart to get to the back of the instrument cluster from above.
Tony
<font size="1">1981 2L FHC Cavalry Blue - very original & not rusty!
1980 3.5L V8 DHC Regency Red - on the road again.</font id="size1">