I believe that if you leave under 10 years unless it's because you are seriously ill, etc, then the employer is not obligated to pay pro rata. If you were made redundant or (like was my experience) your employer sells their business (so really same as redundancy), they are obliged to pay pro rata.
I guess unless you can prove that you had no choice other than to leave to work at home (as a mum) then they won't have to pay. Which REALLY stinks as you earn it. I am currently at nearly 8 years with my current employer and I need a change, but it seems alot to lose if I leave.