Many thanks to Hibee and ev
Upon rereading the letter I see it was in fact David Welsh. W.S.
The WS had me stumped but today I've learned that it means Writer to the Signet, which in everyday language means a solicitor or notary.
The William Maitland referred to in the letter could well have been William Mordaunt Maitland, a legal eagle of his day. He was twice married and each time to daughters of Reverends. Peerage.com shows there was a Reverend Vardon...wheels within wheels.
My letter was written on 28 June 1843, and if you google this date you'll find the scan of another letter written by a young girl in America. The US letter is very clear and easy to read--it looks as if it were written today, whereas Marian's letter is in old fashioned copperplate ("ss" written as "fs") and very difficult to decypher and it sounds as if it was written by an elderly gentlewoman.
So I'm beginning to get a mental picture about this matter:
William Mordant Maitland died and, for some unknown reason, left a goodly sum to Marian Vardon; the solicitor David Welsh managed this money for her.
Marion Vardon, an elderly lady suffering from rheumatism in the Edinburgh cold weather decided to splash out some of her newly acquired wealth and go to Weisbaden to see if the thermal waters would ease her ailments. While in Germany, she requested David Welsh to transfer some more cash into her account, which he did, and in this letter she is thanking him.
152 pounds in interests meant she surely was wealthy.
How's that sound?