As stated above, this mission is impossible at least on medium difficulty. I managed to defeat the ships that attacked initially, storm the chapel, with two companies of marines and about 6 boats of crew from three different ships but was quickly surrounded and annihilated. Securing the escape cost me my remaining survivors. Not only that, that big ship laid up to the south then attacked my vessels which were already damaged in the previous action.
How can this mission be completed? Who could pull off such an amazing caper?
I even tried, in another attempt, landing far up north on the shore, carefully wooing a division of their infantry into gun range from my ships and whittling them down for hours and hours by disembarking small teams of men and re-embarking, then landing my marines to mop up but was eventually flanked by a relief force of significant size coming down the coast from the north.