Worcester Greenstar 42cdi - Weird Error Code for Heating

I'd be really grateful for advice here.

I have a Worcester Greenstar 42cdi that is just over 2 years old - Pressure is okay, pipe outside is lagged and covered. The water heating side is working okay.

The central heating does not seem to come on as usual in the mornings.
If I turn off and turn on again - then wait about 2 mins - I can use the controls to turn the heating to 'on', which works for about 20 mins then stops. Then I do it again and it will work fine all day but the display is showing a symbol that alternates between the symbol and the current temp. This is not in the book and when I google I can't find it. Error as follows:
-| |-

The following morning we have to do the same again. 3 mornings in a row we have woken up to 16 degrees! I think its related to the cold because it started when the weather got cold. In fact I used a kettle to ensure the pipe outside was not iced up as this happened last Winter and this is how I first got it to start (on the coldest day), but since then I havent had to do this to get it started.

The pressure is about 0.5 when completely switched off but it is about 1.8-2.0 when switched on and running normally.

Is there anything I can do or do I need to call a plumber out?

Increase the pressure to between 1 and 1.5 and you will probably be ok.
The symbols are for syphon filling, they will always appear after the power is turned off, and stay there for about 20 mins.

Hi
The display code isnt a fault it happens when power is turned on and off to the boiler, putting it into a 15 mins minimum heat setting , it explains this in the installation book under 'commissioning, starting system'.

The fault could be wireless stat or programmer or being that it is cold the condensate pipe ouside froze, like you mentioned. This should be in 32mm waste pipe to stop freezing
Your pressure is also low so i should top it up to 1.0bar when system cold
hope this helps Mat

Thanks for the advice. On further investigation (armed with my new knowledge - courtesy of you)....
My plumber has mistakingly (ahem - being very polite) put 22ml copper pipe instead of 35 ml waste pipe outside! So even though it's insulated, it's still getting too cold and not clearing the condensation properly.

Problem identified. Now, just need to get it sorted.

Thanks for your help. Very much appreciated.