We need a new system boiler for our house. Our local installer is likely
to
recommend a Glow-worm 30sxi but I have heard good things of the Worcester
Bosch Greenstar 28HE system.

Sorry, in a bit late on this one.

I have a Bosch Greenstar 28HE. It is very good. It just works.

The (relatively) new Glowworm range, which includes the 30sxi is also well
regarded, unlike their previous condensing range, which was not good.

I would be very happy to have either these boilers.

Does having the programmer internal to the boiler better suit a modulating
burner (i.e. does the programmer provide more variable control - not just
on/off - in this circumstance) ?

I have a feeling that the Bosch solution uses an analogue temperature sensor
so could conceivably apply variable control. I am not sure of this, or
whether the algorithms used are significantly better than a on/off control.
My system is substantially subzoned, so would not have been able to use it.

Christian.