As part of testing I thought I'd compile some existing projects to see what happened (after copying them from my development environment of course), plus there is more data in them to see with the debugger.
One app that uses StringBuilder quite a lot changed behaviour, the reason took me 2 mins to find using the Debugger, great stuff.
Basically in the app compiled under B4A V1.3, amongst quite a few string building routines I had what amounted to
StringBuilder2.append(Stringbuilder1)
This wasn't intentional but worked so I hadn't realized I had an error in the code as appending Stringbuilder1 returned the string it contained.
The code should have been (and was the case in all the other occurrences)
StringBuilder2.append(StringBuilder1.ToString)
In 1.41 using append(StringBuilder1) this way now returns '(StringBuilder)' and then the string hence the change in behaviour.
I wanted to post this in case anybody else has the same issue.
Steve