Hi Simon,
I have just been through this at our Trust. We too were on Whitley Council rules until around Feb of this year.
Turned out our Trust had negotiated a new on-call payment policy with the Union etc 2 years earlier but we had been forgotten about!
We are no longer paid by sessions but by the hour. Whenever the workshop is closed then the on-call is active. We do 100 hours a week on call. You get more for weekends and BH per hour of course.
If you want me to send you a copy of our policy then just send me an email.
Ian Wilde