Problem with Ravenheat LS100T combi boiler with built-in timer?

I have had the above boiler for about 2 yrs now. But in the last 4 weeks, it has not been working perfectly.

Whether heating on or off, when I turn hot water on, it gives me sometimes hot or cold water. As the pilot light goes on and off when the heater and/or hot water on, the plumber changed the sensor, but still not working.

In the last few days, the heater seems having problem too because it doesn't give the heat what it is supposed to do. When I touched plumbing (bronzed) pipes, they are not hot at all, not even warm. They used to be quite hot to be tocuhed.

And this morning I noticed that when I put the heater on manually, it didn't start its function immediately. It waited for few minutes before it started, which I think this was wrong because normally it is on immediately after I pressed the on manually.

When the heating is on, I am hearing noises and sometimes they are cracking noises. These noises sometimes on for few seconds and then off again. It does this on and off all the time when the heater is supposed to be on or when I turn the hot water tap on.

I have undefloor heating throughout the house.

Can you please help me what is wrong with my boiler and how to fix it?

Thank you very much.