Thanks Matt, it's been a while since I messed with this stuff but it's funny how it all makes sense when someone tells you the answer. Works a charm, but...
Alas, the situation is more complex than I had thought...
fieldA contains serial numbers for a device and gets it's value (by means of a Default Value, as said previously) from a secondary datasource. So when the 2DS changes the field should update.
FieldB is a DDL of the device types. Only ceartain device types have their serial number available in the fieldA 2DS. FieldB has other rules on it that make reselecting the device type arbitrarily (to trigger a rule to update serial numbers) very bad.
The problem with the current (Default Value) situation is that, for devices that do not have a serial number in the 2DS, the Serial number will default to blank when the form is opened. So where a serial number is not in 2DS and you enter it manually, then reopen the form... it's gone again.
Having re-examined this situation it seems to me I'll have to look at filling this field with code in the OnLoad event.
Any further thoughts/ fully worked solutions? ;-) hehe