Reset F.28 error on ecoTEC plus combi boiler?

We turned on our boiler yesterday, unfortunately we forget we had also turned of the gas.
When we realised this we turned on the gas supply. Unfortunately the boiler now displays error F28 – no/failed ignition I believe.

I have tried resetting, pushing the reset (flame with diagonal bar) button for various lengths of time also after turning of the power.

The boiler cycles through the start up sequence (apparently successfully) but then stops and displays F28. I presume this is because it went though too many auto ignition failures when the gas was off.

How do I reset the the fault?