vokera linea combi boiler problem?

have just moved house, when the boiler is set to hot water only. when the water temp drops below 40 on the display the boiler fires up and heats the water to 60. but there are no taps on or dripping, after 10 of 15 mins the temperature drops below 40 and it fires again and heats the water to 60 again.
is this supposed to do this or is it a fault, if it is a fault what is the fault?
another problem is that when running a bath if the hot tap is turned on so that it is running to quick the water will go cold, then hot then cold then hot etc etc
but if i turn the tap down to run very slow it stays hot.
a friend said it sounds like a dodgey thermostat,
i am more concerned about the first problem as i have noticed i am using a lot of gas. i am on a prepeayment meter with edf an have done £45 in 9 days. where as at my previous address it was more like £20 to £25.