Are you playing out the 1643 Lansdowne - Roundway Down campaign by any chance?
Yes we are, as a matter of fact.

We are using scenarios from this book (
link):

I think the exact location of the battle is somewhat conjectural but the battle is not fictional. There are a few 'what if' scenarios in the book; in fact the next one in line is, but I am not sure if we will be playing it as it is based on the assumption that the Royalist win Monkton Farleigh, as they did historically.