The message
from Andy Hall contains these words:

Is there any danger of them backing off so much that there's insufficient
flow to cool the boiler during pump over-run conditions?


There is a control to set the operating speed/power.

In any case, though, there is not a need for a very great flow for
bypass purposes - only enough to stop the water boiling if it were to
otherwise stop flowing. Also, remember that this is more of an issue
with cast iron boilers than those with low mass heat exchangers

Boiling is not really an issue for me. Firing the overheat cutout is. I
don't have an Alpha but I occasionally get the boiler isolated by the
overheat stat which could be a problem if it ever happened when I was
away from home in the depths of winter. However AFAICT it only happens
when the boiler is heating the DHW shortly after the central heating has
turned itself down for the night and then only so occasionally that I
can't be bothered to bugger about with the flow rates though the heating
coil and the automatic bypass.

--
Roger Chapman