Did you get a genuine up to date Ford HCV? (FINIS 1451981) The pattern parts from ebay and the like are known to be tempremental and stick.
The latest Ford batch code starts with 7N21 and not the 98FU that were assocated with the faulty Ford ones. The 7N21 has a slightly larger bore size than the old 98FU.

If you're sure it's not the HCV, then the heater control panel may have failed as I've seen that in the past. Plenty of them about ebay or down your local breakers.

Good luck!