Solved it...
Modified webvr_enterbutton in webvr.xml as below:
<layer name="webvr_enterbutton" keep="true" vr="true"
style="webvr_button_style"
parent="group5"
html="ENTER VR"
align="top" y="0"
autoalpha="true" alpha="0.0"
onclick="webvr.enterVR();"
/>
Then created a group in my xml file as below:
<plugin name="group5"
url="../hotspots/transparent-45x45.png"
align="top" x="0" y="0"
keep="true"
/>
I can then control if the vr button appears for each scene using the onstart function:
<scene name="pano2" title="pano2 " onstart="showname(); set (plugin[group5].visible,true);
">
or
<scene name="pano2" title="pano2 " onstart="showname(); set (plugin[group5].visible,false);
">