I have a standard 2015 4rt and would like to upgrade the rear shock spring as I weigh 102 kg

I have from a previous beta Evo a heavy duty spring which is the same diameter as the Montesa but is 13 mm longer however the standard spring was wound up around 15 mm so would there be any reason why the longer spring would not work if not wound up to same length as previous

I agree, its all about if it sits properly on the collars and once adjusted for sag that it clears everything.  If yes for both it should be fine.  (operative word being "should")  

It's not just about length, you need to know if the spring rates are different otherwise it could go coil bound and lock out before hitting the bump stops.

I'd recommend fitting a firmer spring though. I'm the same weight as you and fitted on to my 260 repsol and it made a great improvement, I wish I'd fitted one to my previous 4rts.

Thanks chaps. Beta spring fitted just fine loads of space and even though a little longer feels good in the garage so roll on Sunday for the true test.