There used to be some info on this in the Employee Handbook but it has been taken out. I think the only place to get the definitive answer is in the Staff Travel Manual which is not available on the internet (as far as I can see). "It will be made available on request to your manager."
If anyone has discovered the answer on the groupworld site could they post the location.
From the old book:
SRC's avail after ten years if within 10 years of Normal Retirement Age (NRA) which is 60. NRA is 65 for those who joined after Jul 07.
Only applies if not employed by another company which provides travel benefits.
If you have completed ten years and reached NRA you are also entitled to ZED fare tickets as per the OAL agreements.
Again, I'm not sure how up to date this is but if someone has access to the official manual please post.