On modern spelling, it would be pronounced "Swin-sted". Assuming the spelling is accurate for period, it would most likely be pronounced "Swin-eh-sted".

The rule for Englisc (pre-Norman) spelling is that all letters are voiced. Hence, cniht (the original of knight) would be ku-nite, & write would be wu-reet-ah.

Swine-sted is extremely unlikely as a pronunciation. Although you are suggesting early 13thC for period, the chances are that, for that name, the Englisc is still current.