Worcester bolier - water goes cold?

Hi there
We have a Worcester 281 RSF boiler, in a three bed semi in a hard water area.

Lately we have found that the hot water will suddenly go cold. We have noticed this in the shower, bath and kitchen. The water will be nice and hot, suddenly drop to ice cold and then eventually go back to hot. The cold water intervals are becoming more frequent and lasting for longer. This morning I ran a bath and when I went to test the temperature it was ice cold! I then tried the shower and basin - both cold. Went downstairs ran the kitchen hot water and then it came right. I was able to have a shower but it still went cold at intervals. I had a look - the boiler pressure is at 1. We did have it flushed about a year ago because the hot water pressure had become increasingly low.

We will call in a Corgi registered plumber but any ideas what we are dealing with and possible costs? ( We are from Australia so boilers are totally foreign to us (:

Thanks for your help.
Michelle