BIASI M96.28SM/C

I'm looking for a bit of advice. I had this boiler fitted last year and straight away noticed it wasn't working right. Hot water is fine. When the heating is on the burner is constantly going on and off. The pump is running constantly while this is happening. This means that the rads never get up to temp so that the room is warm enough to switch off the room stat. To me that's not right. My old boiler would fire up and keep going untill the room stat tripped and burner and pump turned off until heat was called for again.
I did report this to BIASI who sent out a firm. They were useless. First time they told me it wasn't powerfull enough for my house. I only have 5 Rads. Next time they came they drained some water from the system, but didn't top it back up. Then he said it's probably the room stat as it's quite old. Now I suggested what I thought was wrong both times they came but that was ignored.
Looking for opinions on what you think is wrong with it. Thanks

I'd start with a CH thermistor with this one. It was a fairly common problem where the system wasn't flushed properly.

if you hot water is fine then i would go with poor ch flow. what is your system pipe in(microbore or 15mm?)

I would work out what your house really needs kw wise and range rate the boiler down to that(it will help with the short cycling) it is in your manual how to do it. then see how dirty your system is. mostly it will be full of black crap so would need cleaning.