Ineffective central heating and hot water

I've recently moved into a new (to me) house that has a SysteMate 2000 fitted with an Ideal boiler. The house is about 10 years old and has had this system from new. It is over three floors and has 17 radiators.
Now the weather has started to get cold we noticed one of the radiators in the downstairs lounge wasn't getting hot at all and the radiator in the main bedroom above was only getting luke warm while some of the other radiators were getting hot.

We've had a local plumber round who flushed and cleaned the system and then diagnosed a blockage in the return pipe of the lounge radiator and subsequently re-routed this upstairs to feed into the upstairs circuit - the house is on 10mm plastic pipes. This radiator now gets warm but still doesn't get hot, the one upstairs is still only lukewarm.

I don't know whether this is related but it seems that the boiler shuts off before reaching the temperate on the thermostat. I've tried turning the thermostat up to full and the boiler just seems to be idling without firing. I don't really want to get this plumber back without first trying to understand a bit more about the system. Looking at some of the posts on here it would suggest that it may be the systemate thats at fault and not the pipework.

We also noticed last night after both my partner had a shower quite soon after each other (within an hour - fortunately I got in first!) that the second shower wasn't getting as hot as before. I've not noticed this before so don't know if it's related to the recent work.

Any help or advice to diagnose the issue would be greatly appreciated, the missus isn't very happen at the min.

HI

The problem that you have described does not suggest that the fault lies with the Systemate, but with the Ideal boiler instead.
You have said that even with the boiler turned up to maximum (which is where it should be at all times with this system anyway) the boiler only seems to idle without firing, therefore the system will not function correctly without adequate heat within the Systemate.

we have the same setup but different boiler, problem is just the same with random luke warm radiators and sometimes just half filling a bath before the water goes cold..... annoying as hell...

i'm at the stage where spending 5k is the only way to sort this, have you bottomed the problem ??