Vaillant Turbomax 828e hot water intermittent problem (CH works fine)

Hello,
There appears to be a fault with my boiler.
The heating for the radiators & washing machine works fine.

However the hot water from the taps is the issue.
When I turn on a hot water tap sometimes it heats up the water and sometimes it does not.
It looks like the ignition (for the hot water) works intermittently.
When the ignition does work, the water heats up to a specific temperature (50 degrees) but then cuts off immediately and the water goes cold again.

It's displaying these codes at different times:
s.24, s.27 & s.30

Looking on the forum for similar problems it looks like it might be one or several of these:
-flow sensor
-mesh filter
-hall sensor
-flame detection electrode needs adjusting and/or the minimum burner pressure is out of adjustment.
-diverter valve
-fan assembly

Anyone have any specific idea what it might be?
Also would it be realistic for me to try to fix any of these problems, or is that totally unrelaistic?

Many thanks

Ok,
I have set the hot water dial to almost maximum.
The water heats up to 60 degrees, then it stops heating, the temperature drops to 35 degrees and the boiler starts heating the water again.
And it keeps repeating, = heats up to 60 degrees, then stops heating until temperature drops to 35 degrees and starts heating again..

same code is displaying s.30

That would imply a blocked secondary heat exchanger!

Tony

Definitely not the diverter valve?
Can the secondary heat exchange be cleaned? Or is it better just to replace it for a new one?
Where would be a good place to source a reasonably priced one?

I always clean them but many opt to buy a new one.

Why suggest the diverter valve?

Tony

Ok brilliant.
I would be interested to know the difference in price between cleaning it out and fitting a new one.
What would be a reasonable charge to clean it out?
And what would be a reasonable charge to supply & fit a new one?

The reason I mentioned the diverter valve is I was reading on a different forum (I'm not sure if I'm allowed the mention the name)
I thought there was a person there who had a similar problem and they replaced the diverter valve.
However after re-reading it appears I was reading the wrong post.
In fact the correct post said the same as you:
"your problem most likely to be domestic hot water plate to plate heat exchanger partially blocked or scaled.
Usually divertor valve problem is no hot water at all and boiler will not fire."