Andy and Polly M, and David and Gaynor F have both been recently and can thoroughly recommend a visit. You can park along the small lane alongside the A11, then cross the footbridge then walk a short distance along Fleam Dyke (see instructions at the bottom of this post).
However, in the mean time, I thought it would be nice to show a 'virtual trip', and what you might expect to see there in terms of butterflies:
How to get there - from the A11 heading north - turn off the A11 at the Fulbourn junction, and at the junction turn left then immediately right onto the small road alongside the A11. There is a small car parking bay just after the footbridge).