My daughter had just bought her first house (despite my protestations
that it's not the right time etc etc) and it has the above combi. The
house is a 1969 built 3 bed "modern" (ie pretty small and poorly
built) terrace. It has 7 single panel rads. I've had an initial stab
at balancing the rads.

The boiler is struggling to heat the house although it does get there
eventually. It appears to operating in either on or off - boiler
fires and the flow pipe get almost instantly too hot to touch, runs
for a few mins then shuts down again. It stays shutdown for a period
(I think I read somewhere it won't restart for 3mins) and in that time
the circulating water has cooled considerably.

Then it re-fires and the process repeats. Eventually the the whole
system gets warm but I would have expected the boiler to run until
that happens.

Either it isn't moduating, or the system can't dissipate the heat fast
enough even with the burner on its minimum setting. Is there any way
an amatuer can check if the boiler is modulating?