..........any chance of getting on to the HRMS system & speaking to them?.
....The people on the phones at HMRC are certainly better trained to be much more friendly than they used to be in my day, but I would advise getting CAB or professional advice before speaking to them.
It's not a matter of illegal tax evasion but of legal tax avoidance and politely challenging a tax demand. It first needs to be established beyond doubt that the tax is even due at all. It's best to realise that the HMRC taxperson is not on your side but has the job of maximising the tax revenue, so that's why I wouldn't talk to them without first getting advice.
Put you and your family's best interests ahead of HMRC.
If the tax is due, I think you need to stretch out repayment as long as you can and even negotiate that length of time. I take the view that HMRC may be at fault in allowing this build-up of tax to happen, but they won't agree or admit to that.
I can only offer this advice based on my own, not inconsiderable, experiences with tax people.
EDIT: Just read your post:
Thanks, Robin, I am in the process of dropping an e-mail to my personnel department to see what comes back.
....Very good move (it is a bit like a game of chess!) - A Personnel dept will have experience and will be 'on your side' to help you in your best interests.
I would use your service overseas period to delay this if you can. HMRC can wait 4(?) months without any problems!