I'd start off with "Mmmphmuumumutghfgdmum" probably and then say something completely starstruck like "You way off the beaten track, mister - you shure got a purty mouth....".
Really, tho' what would you say ?
If the pubs were open it would have to be "You've given me over 20 years of pleasure, the least I can do is buy you a pint" or something equally fawning..