Vokera Compact 28. Towel warmer replaced with Rad today by friend.. System repressurised. Now 1 rad stone cold. Lockshield and TRV fully open. Tried closing down all other rads to force through and boiler switches off to standby mode (flashing green light). I presume air in the system. Tried bleeding rad but no air. Open other rads and boiler turns on again. Help?