This is a bit of a misnomer now. It, arguably, enjoyed a good peak service to and from London immediately post Thameslink in the form of non-stop 387 services, but in reality this was a double-edged sword as going into London Ashwell passengers would be the last on, so stand a slim chance of finding a seat.
All but one of the 387 calls have now disappeared, being replaced by the second Cambridge/Brighton service. So Ashwell now gets 3tph most hours give or take. This compares disfavourably with Letchworth which gets 4tph off-peak and (from memory) at least 7tph peak. One fast service to London per hour is still a pretty good service for a relatively sleepy station.