Last year, I took voluntary redundancy to go and care for a relative. Things have not gone well there so I’m going back to paid employment. The customers of my last employer gave me glowing feedback so I’m approaching my last employer first. I’d appreciate Dopers’ comments on how best to do this.
I would call to find out when the person you need to talk to will be there, and then go in person. If they are having any reservations about rehiring you, it will be harder for them to tell you “No” when you are standing right there. If you’ve proven yourself as a valuable employee, they will likely find something for you to do, given your altruistic reason for leaving in the first place. Good luck!
Most employers want to take back employees in your position. You left on good terms and have training that may need a little updating at most. You just need to indicate if they should only consider you for your original position, or which I would suggest you want to be considered for a number of positions if they are ever available. Don’t expect your original wages will be paid automatically. You shouldn’t accept a ridiculous wage starting at untrained newbie wages.
If there is an opening, this is probably a slam dunk. Really good employees are rare enough and also people like to stay in their comfort zone.
Even if there isn’t an opening, some companies will make one in order to not lose a good resource.
I would start with my old boss and see if he knew of any openings.
Good luck!
I have several times had employees leave then ask to come back, albeit in different circumstances from yours - they take another job then realize they actually had it pretty good with us in the first place. As long as they were good employees originally, I take them back at the same salary - I will not “reward” their leaving by giving them a higher salary when they come back. Sometimes I will not take an employee back - people who were below-average contributors but not poor enough to get fired.
I would prefer to receive a phone call rather than email or letter, then set up a meeting if appropriate. I would think given the circumstances you describe, your former employer will be pleased to hear from you.
I guess one question is if the redundancy involved a large payoff. If you sense resistance around this point, you might want to suggest “repayment” terms (e.g. a time-limited reduction in your asking salary) to offset it.
It wasn’t that large. 
But part of the terms were that they do reserve the right to claw it back. I’ll have to get them to not claw it back.
Another issue is the prospective department of my employer being in the US and me being in the U.K… This may obviate the clawback issue, of course.