I've spoken with a colleague in CS who has some insider knowledge of the EPSRC, and he said it's possible that they won't care what field the software is written for as long as it's science. He said that at the very least it's worth emailing them to find out whether what you'd propose to do is within the remit, because it doesn't seem clearly excluded.
He also said that the early career computer scientists he'd spoken to seemed less than interested, so it may be that this is undersubscribed, especially if this is its first year. So perhaps it's worth finding out more, if this is something you might be interested in (and/or if you can justify the software as being worthwhile for more than astrophysics?).