Alright mate, how you doing!
Your brooks saddle shouldn't sound like plastic. It may be that it hasn't been used for a long time.
when you press the leather with your thumb or put any weight on it, are there any cracks in the leather? Or does its shape not deform at all?
If the surface is fairly shiny it may have been well treated with proofide or something in the past, this may have gone hard.
Try putting the saddle in a thick platic bag and dunking it in some warm water (don't get the saddle wet, just try and warm it up), or use a hair dryer just on warm setting to try and get the old leather warmed up. Once it's warm, give the leather a bit of a rub and see if wax comes off, if so you probably just need to rub this wax in a bit (the heat from your hands will soften the wax up).
If there are small or large cracks in the leather or it feels brittle (feel the edges see if they bend), then you may have to be a bit more careful. Try rubbing a little proofide in topside and underneath. Keep the saddle reasonably warm for a few days hours and see if the leather gets any more supple. You may have to do this several times.
Even so, if the saddle is cracked and the leather hasn't been looked after, no matter how much wax you put on now the leather structure may have disintegrated - leather does biodegrade, eventually. If the cracks get bigger or the saddle gets considerably baggier as you ride it, you may be out of luck.
(Cop-out clause: I'm just guessing all this from past experience with walking boots, i've never had to coax an old saddle back to life, but it's leather so probably behaves the same).