Please would someone advise me as to whether I can extend my Tourist Visa (I have a double-entry) at the Kanchanaburi Immigration office, rather than having to travel to Bangkok?

Thanks. Duggsie :-)

You can extend your permission to stay before your initial 60 day permission expires at any immigration office, although they may want some proof of your address (hotel or whatever).

You need to watch both the expiration date printed on the visa sticker to make sure you make your second entry before it expires and also the permission to stay date stamp given you by immigrations on your arrival in Thailand and when you extend at an immigrations office. You can do the extension a week or two before your permission to stay expires. You won't lose any days since the extension is added on to the end of your current permission to stay. But when you do the border crossing for the second activation you start over with a new 60 day count.

Visa expiration dates are not extended. Permission to stay in-country is.