Am I worried about nothing? Central heating

lesae

I have a 6 year old Viessmann boiler with a hot water tank. In the summer we had an issue in that the central heating was coming on with the hot water. Plumber came out and sorted that by replacing the 3 port aculater. 
When it came to turning the heating on a couple of weeks back, i noticed that it doesn't turn off even when at temperature . The only way I can get it to stop is to turn the boiler off and on again. Plumber has been out and says it's something to do with the voltage and Hive and not the 3 port aculater he changed. Hive says it's off when it's very much not. He came again the other day and then mentioned the valve and I think he's planning on trying a different one. I'm a bit lost with what he is saying too be fair. 
In the meantime my radiators are scalding hot when I turn the central heating on, thermostats on them don't work. It's set to 66 on the boiler so this shouldn't be happening. The other day when I turned it back on after having the heating on for just over an hour it said it was at 78 when it came back on although 66 when I turned it off. Pressure tonight after an hour or so was 1.4. 
I've read some things online and now I'm in a complete panic that it might explode or I'm going to have bigger issues. Getting the plumber back is hard as he's busy but I'm too scared to have my heating on for long. 
Am I right to be concerned about having it on for long?
The hot water doesn't seem much hotter than normal when I've had it on for 45 mins, just the radiator apart from one upstairs which is barely warm.
Many thanks in advance

molerat

Different things in play here.  

Some boilers seem to do a v hot cycle then settle down to the correct temp so likely nothing wrong there - I have seen it on my daughter's.  The boiler won't explode as that sort of temp is well within its "normal" range.

The rad stats not working is another thing.  Take the head off and see if the pin is free to move, likely sticking.

If he just changed the actuator head it could simply be the valve itself is sticking, a more complicated job than the quick fix actuator change - or as in my daughter's case the actuator was wired in incorrectly !

twopenny

Interesting because that is almost exactly what happened to me last year.
Had new actuator fitted and the rads and boiler overheat.

Had 3 people look at it, 2 threads on here and can't find what is wrong.
Fortunately at the moment an hour's heat is retained in the house for quite a while but that's not a long term solution so following with interest.