Hi Aydat
I am no expert on this but have you got contact details for other ex employees to see if they received this letter?. Even this would not prove anything. If they have your letter could have been lost.
Going by logic I would have thought this was not valid.
In theory if this was valid an ex employer could introduce a rule saying you are not to work for the employer you work for now 20 years after you left for example.
If you were in a union I would contact them or the Citizens Advice Bureau.
John



