BIASI Combi produces heating but no hot water?

I am at my mums and for weeks it would appear she has been living with this problem. The BIASI Combi runs the central heating but when hot DHW is called for the boiler runs for 10 secs and then goes of for 5 secs, you then get hot water for a few secounds then cold. Makes showering a bit tricky. I am soon to leave for home 270 miles away. Any ideas as to were to point to and leave her with some information for the engineer if she needs one. Thanks