Alde overheat error

Thanks for reading.

I recently changed the coolant fluid in our Bailey Approach 540 Alde heating system. The MH was tilted to facilitate bleeding, which appeared to go well as we had no issues in our first outing following the change. However, I am now getting a red overheat error on the 3010 display.

Having rooted around the net and youtube, I am advised that this is most likely an air lock. In the first instance the electric side of the boiler failed. This was resolved by a "reset" and removal of fuses on the boiler and the 12v feed. Unfortunately, the red overheat screen remains. 

I would like to resolve this without having to resort to my local service shop as they messed me around for months with a previous issue.

Words of wisdom please.

AH