Thanks for a very thorough answer. The dereferencing to get at the coderef had got muddled. Your approach of defining the coderef separately is much clearer. I wasn't sure that defining a coderef that calls a coderef was necessary, but set out like that it appears to be.

As a bonus, you spotted a bug that I was still struggling with - $tkobjects{'stringlabel'} should have been a child of sourceoneframe or sourcetwoframe, so when I came to destroy the frame and replace it the label didn't disappear.

Thanks also for taking the time to dig through the code.