Hi Micha
If you have received an assessment date then it is the assessment services that you need to contact. You can try ringing them and asking them to do a paper-based assessment (but very few are done). They are unlikely to change their mind unless you have support from a GP or consultant that confirms that for you to attend an assessment would cause you 'overwhelming psychological distress' and is a risk to your mental health.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
