I recall reading somewhere that many people involved in the English Civil War (ECW) had fought as mercenaries on the continent. This apparently affected the fighting in the ECW itself, as the officers insisting on adopting tactics they'd been trained in under the Swedes.
Is this fanciful rubbish or is there some truth to it? I can't find much about battlefield tactics for the ECW beyond general movements on the field.
This is absolutely correct. The ECW ran nearly concurrently with the 80YW (between the Imperial Spanish and the Dutch rebels), and the 30YW (between Catholics and Protestants). During these wars, several mercenaries from England, Scotland, Ireland, contributed mostly on the Protestant / Dutch side. Many returned in time for the ECW to break out.
I was going to start writing on this subject, but I can't do justice in lieu of this great website:
http://bcw-project.org/military/tactics
It is well written and well referenced beyond what I can myself write, so enjoy!