Halstead High Ace Boiler HW & CH Problem?

Hi, I am at my wits end and need help as winter set in with 3 children.
My boiler has played up for many years and had enough. My boiler failed to work for 2 day so I called a Corgi guy out last week. Sad to say, the boiler sprung into life when he turned up and checked everything and said all was working well. Now HW & CH are intermittant and comes on when it wants to!

Example: Failed to work last night. HW worked in morning. It has good flow and then reduced flow by 70% but was hot. 5 mins later I flicked switch to HW & CH and the HW failed to come on! Still no CH.
The past behavoiur is: HW on but running cold then luke warm and then it is a hit and miss if it will be hot or not!.

Please help.