Limited / no hot water on my Boilermate 2000

Hi All,
This is my first post so please be gentle. I've recently moved into a 6 year old property that has a Boilermate 2000 installed. The property was a repossession and was 'winterized' before it was boarded up. When we moved in I had a plumber 'de-winterize' the system and get things up and running again for me.

However, there appears to be no hot water. The hot water pump is running and you get about 5 secs of hot water then it just runs ice cold. The plumber has de-scaled the Heat exchange and said the system should be up and running ok, but its not!

On a side note I have a 'switch' on the front plate of the boiler where you can manually select 'Emergency hot water'. If I select this I get instant hot water to my hearts content. Which makes me think the pumps can't be at fault.

Oh, and the central heating is running perfectly.

Does anyone know where the problem maybe? A quick search of this forum suggest that the ACB might be blown or to replace the DHW sensor?
Sorry for the rant but I'm at my wits end and with all the bills with recently moving I can't afford to pay a plumber to come out and charge me if its a simple fix or something I'm doing wrong.
Cheers

Start with DHW sensor