Boiler switches on/off every minute when heating hot water?

Hi.

I need some advice please. I'm renting a property which has an Ideal Classic LX FF & FF Fanned Flue Gas Boiler model FF230 installed.

A couple of months ago the boiler failed to fire and we had no heating or hot water so a engineer came and replaced a broken fan. Shortly after the boiler keeps switching on and off for a minute every minute until the timer turns it off around 10pm. This seems to only happen when heating hot water, not when central heating turned on. The engineer is fobbing me off saying this is expected behavior as the boiler is topping up the water temp as it cools, but I'm finding it hard to believe water temp could drop so drastically every minute that it has to keep turning itself on and off. Then the engineer said we are using it wrong anyway and we should only be using the gas to initially heat the water then use the electric immersion heater as required for more hot water i.e. shower etc. That would just be too expensive to run and just sounds daft to me. I can understand using electric when you want the water heated quickly or if gas boiler failure but not for topping up. I think he hasn't fitted something incorrectly or something else is wrong.

Now my agent is saying if there is no fault we have to pay the call out charge.

Any suggestions?

Thanks