Happy,
Sorry to be slow off the block on this one - I thought that the whole paying back of FRIs was so common now that the relevant Service pay offices were well versed in assisting those wishing to do this.
Anyway, check your PMs - although still serving I have paid mine back and just waiting for the right job to jump. 50k certainly wasn't enough for me and to be totally honest the bean counters are dreaming if they think that 100k is going to tempt me to stay.
The average divorce cost more than that and the quality of life at the moment is no where near 100k.
Anyway, I digress - give the number/name a ring in the PM for the tax office, really helped me out and very friendly indeed. All sorted in 2 weeks. But you do have to start the ball rolling by paying it all back in FULL. And make sure you get a receipt and covering letter to say that you have paid it all back.
Good luck!