There seems to be nothing between the W-Bosch 28HE and Glow worm 30SXI;
prices and features are all similar BUT both have fixed-speed pumps
rather than self adapting pumps (e.g. alpha) - is this likely to be a
problem with a 3 zone (+ DHW) system?

No. I have a 3 zone + DHW system running from the Greenstar 28HE and it runs
just fine. Zones are upstairs/downstairs/kitchen/DHW. I intend to add a
couple more zones. I do have an automatic bypass as well, though, which
should reduce noise when running a low load.

Christian.