If you do not agree with the tax charge thenthe only way you will get matters resolved is to advise HMRC in the UK that you do not agree with the charge and this will result in it being referred back to the tax authorities in Germany to deal with your appeal.
If this was PAYE tax e.g. contractor working in Germany with some salary on payroll and other amounts by loans or "off record" then you should also consider what is happening to other employees who were with the same employer and why if it was that tax was not deducted why have the German authorities not gon after the paymaster.
Suggest you contact a tax specialist if you are in any doubt as unless you appeal and have the matter referred back to the Germans then HMRC could end up taking legal proceedings.
Of course you should have dealt with it previously given that it relates to over 8 years ago and you would need to ensure that your rights are maintained throughout. Of course given the timescale there might just be a suspicion that you have been ducking and diving and trying to "evade" or "avoid" your tax liabilities by fleeign the country before they caught up with you..... LOL
regards and hope that this helps
bill@wamstaxltd.com
www.wamstaxltd.comregards and hope this helps
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