Hi commited
We are quite often asked this question.
Always send the most up to date letter you have, if you do not have an up to date letter about your condition which is on going, then send in the most recent letter you have in regards to that condition and explain on the form why you are sending in that letter i.e " on going condition no change in medication, reviewed periodically by my doctor"
Any letter is relevant if you still suffer from the condition, at this moment in time it is a nightmare to try and get to see someone face to face to get a more recent support letter.
We have had members on this forum who have sent in letters over 10 years old as the letter was still relevant to their condition. Letters are out of date when either your condition worsen or improves.
Gary
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
