Hot then cold then hot then cold (etc) water!

In the morning the water in my flat keeps switching between 30 seconds of hot and 30 seconds of cold water. There is no green light on the front of the boiler (though there never has been, and it has been working fine this year). I opened up the front and saw the red light flashing twice every second (which I understand means the system is ok). The tank at the top is full.

I did have a similar issue around the same time last year, which i solved by filling up the tank, so am unsure what is causing this this time.

Any thoughts would be much appreciated.

When you run water from the hot tap.
Can you hear the pump running?
Mike

There doesnt seem to be any different noise from the boiler when the tap is running.

Well you will need to get some one out.
Look on the home page of this site.
There are various persons advertising.
May be one in your area.
Or Google for some one in your area.
It could be quite a few things.
And a few checks nedd to be made.
Have you the book?
If not download one for free from Gledhill.
Mike

Ok thanks for the advice. Will look over the manuals.