Jeff, you have to remove the outside flue cover for the gas side to work, and make sure the gas isolation tap is in the ON position
The clunk you hear is the gas valve opening, then a 10 second ignition cycle.
If it fails, it is more than likely just the gas has not purged the pipework, try again,maybe several times.
As for the electric side, it is not instant hot water, it takes about 30 to 40 minutes to heat the whole tank, but if it not heating then it is most likely heating element has failed.
If all else fails get a Truma authorised engineer to look at it.