Compilation failure on 1.5.6r2 works ok on 1.5.4

Apologies if this is not the right part of the forum - troubleshooting seemed most appropriate, and I cant see a separate "compiler issues" section.

I just made the move from 1.5.4 (nightly build, not sure precisely which night) to 1.5.6r2 (not nightly).
My process went as follows;
in 1.5.4, compile and load the code (Mega 2560 target)- OK
download 1.5.6r2 from the arduino site - OK
rename download to Arduino156r2 and move to Applications folder - OK (so as to not overwrite the old arduino image just in case)
open 156r2 and load sketch - OK
Select target board (2560) -OK
build - FAIL, see error below;

Arduino: 1.5.6-r2 (Mac OS X), Board: "Arduino Mega or Mega 2560, ATmega2560 (Mega 2560)"

Build options changed, rebuilding all
/Applications/Arduino 156r2.app/Contents/Resources/Java/hardware/tools/avr/bin/../lib/gcc/avr/4.3.2/../../../../avr/lib/avr6/crtm2560.o: In function __vector_default': (.vectors+0xa8): relocation truncated to fit: R_AVR_13_PCREL against symbol __vector_42' defined in .text section in /Applications/Arduino 156r2.app/Contents/Resources/Java/hardware/tools/avr/bin/../lib/gcc/avr/4.3.2/../../../../avr/lib/avr6/crtm2560.o

This report would have more information with
"Show verbose output during compilation"
enabled in File > Preferences.

Then I moved back to 1.5.4; and re loaded the sketch, and it still compiles fine.

Other sketches compile fine in both 154 and 156r2.
(I tried to post the verbose compile debug here, but it exceeds the forum post limit of 9500chars - I have it if anybody needs it)

Any help or suggestions would be appreciated;

MB

Logged on github just in case that's the correct way...