Why would it be an escape? Any signs of captivity? Smew are an uncommon but annual visitor to the area - there's always several all winter at nearby Barleycroft.
Whenever I have seen them on the reserve previously they have been in small groups. Though I only visited Swavesey Lake today, this female is on her own so I was speculating to solicit views (thank you for yours).
Smew are an annual winter visitor to the lakes & females (redheads) are regularly seen on Swavesey Lake. No reason to assume it's an escape.