Are you sure it is dust on the end lens? Some Mk1 or Mk2 Nikkos did suffer from getting tiny particles on the ret itself. They will be in focus when the ret is.
If that is the case then it really needs to go for repair - taking the end lens off will not solve the problem
PS - I don't know who repairs them - I was lucky and got my scope replaced by the main dealer here in the UK.