Hi Stuart, I see no one wants this one! I assume its a very narrow run between two buildings. How far do you need to get? On the basis that 'prevention is better than cure', I assume you've tried all physical means of removing the debris, such as making up a Heath Robinson device on a pole and drag the crap out, bit by bit? If you are absolutely sure this cant be done, then you may need to 'tank' the inside wall of the garage
http://www.dampness-info.co.uk/tankingslurry_g oogle.htm?gclid=CMLj6enIo5ECFRInEAodtVKEWQ