I may have outsmarted myself here.
Whilst both Bigtimer and the Within-time nodes provide nice elegant solutions - they both have one major shortcoming which i had not thought through - they will not support FUTURE times.
So if in my case i want to project out 12 hours to see where will be the best tme to run heavier loads and/or charge a battery from the grid etc - i have no way of pushing a FUTURE date/time into either of these solutions and have it then return what tarrif that future time would fall into.
Looks like i am back to writing this as a series of arrays unless someone can see a way around this ?
Craig