No problem - half your luck if you can manage a stay in Guernsey.
The Priaulx Library is a good place to visit, I found the staff very helpful. I viewed a couple of very old oil painting at the Library, daresay they are still there, of Roland Touzeau and Marie Marguerite de Grave.
I'd be interested to read the letter you mention, I'd guess it was written by Marie de Grave. Your ancestor, Roland Touzeau her grandson, was mentioned in her will to receive a sum of 50 pounds and the residue of her estate after expenses.
The only notes I have on my database for your Etienne (Stephen) Martin - he was a Captain in the Merchant Navy and died in Havanna, Cuba, predeceased his wife Elizabeth - she was a 54 year old widow when she died at Les Canichers, Guernsey.