The release switch under the boiler solved it for us too! After a recently failed PCB causing a similar thing we thought that had gone again! glad it was just this. Our symptoms were the halstead ace high was turning on making the initial noises but not igniting and then going to lockout.
Our was doing this because we have loads of other issues with the heat exchanger and pressure thingy but it gets us by till it is replaced next week!
Before spending £160 changing PCB. Check the system has correctly been refilled i.e. no air. run off through preassure value while refill open for 2-3 mins. When done ensure preassure at 2 bar and leave for 1hr to settle before retrying.
worked for me!!!!!