The displays are normally accurate for kWh but can give wrong £ values. Maybe the very small kWh of your boiler was a hot tap for a few moments, or even its pilot light if it has one.
@Jen16014 On your first post you said gas was 0.61kwh and 10p. Gas costs about 4p/kwh so 0.61kwh should be no more than 2.5p. It just shows the Display has rubbish costs but correct kwh .
What a good idea Whiteowl, I wish I had thought of that. Jen, there is an Eco button on Bosch boilers to prevent this pre warming. Also Jen, forget the 10pence, it is a bad calculation, the kwh shows it is only using about 2pence a day.