We use it, but don't have an EV. I find it cheaper then Cosy... Think I just added my neighbours EV when asked what I drive.
We run hard for the cheap slots, then add in a couple of hours during mid afternoon to top up as and when required! COP is reduced, think we're around 3.1ish, but I'm comfortably it works out cheaper this way.
The 50mm screed and insulation means the temperatures don't seem to drop.