I'm focusing on Time Attack 2013 and therefore only require a National B (non-race) licence, however for anybody who has a disability and wish to compete in an MSA regulated event it is not possible to apply on the day (as I suspected would be the case) and there are a few steps that need to be taken beforehand.
The following are a few links and contact details that I have found to be extremely useful
BMSAD (British Motor Sports Association for the Disabled):
The objective of the British Motor Sports Association for the Disabled is to assist drivers with disabilities to compete in motorsport.
http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=1&cad=rja&ved=0CDUQFjAA&url=http%3A%2F%2Fwww.bmsad.co.uk%2F&ei=jKnRUJnhOMaN0AWnx4HwCg&usg=AFQjCNEeVwPP9wd4I7n_NA9e2CBcjIzMHw&sig2=TapU3PnelELfgJPo1Z8AzA&bvm=bv.1355534169,d.d2k
David Butler, BMSAD Chairman
<TABLE border=0 cellSpacing=0 cellPadding=0><TBODY><TR><TD style="PADDING-BOTTOM: 0px; PADDING-LEFT: 8px; PADDING-RIGHT: 4px; PADDING-TOP: 0px" colSpan=2>How to Contact the BMSAD
</TD><TD colSpan=2></TD></TR><TR><TD style="PADDING-BOTTOM: 0px; PADDING-LEFT: 8px; PADDING-RIGHT: 4px; PADDING-TOP: 0px" colSpan=2>David Butler, BMSAD Chairman ( see profile in Competitor section )

Barn Cottage, Commonwood,
Kings Langley, Hertfordshire, WD4 9BB Telephone: 01923 265577 E-mail: david@justwebs.co.uk.
Websites: www.davidbutler.co.uk
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David offered some very useful advise and has steered me in the right direction.
MSA Representative:
Sheila Barter -Medical Section Administrator
The Royal Automobile Club Motor Sports Association Limited
Motor Sports House, Riverside Park, Colnbrook, SL3 0HG
Telephone: 01753 681 736 Fax: 01753 682 938 Email: sheilabarter@msauk.org Website: http://www.msauk.org/
I spoke with Sheila on the phone today and she was again, extremely helpful and guided me through the process in detail.
I now have to submit copies of medical reports and other documentation along with a formal letter from my doctor stating the nature of my disabilities and how those disabilities would not preclude my competing in competative motorsport.
The above will then be passed on to the MSA Medical Advisory Panel for review.
If the review go's well there should be no problem with the MSA issuing a National B (non-race) licence.
<TABLE border=0 cellSpacing=0 cellPadding=0><TBODY><TR><TD style="PADDING-BOTTOM: 0px; PADDING-LEFT: 8px; PADDING-RIGHT: 4px; PADDING-TOP: 0px" colSpan=4>The Medical Advisory Panel is Chaired by Dr Phil Rayner
(MSA Registered Doctor, Race & Rally Rescue Licence, Chief MO on the Wales Rally GB,
Consultant Anaesthetist, member of the FIA Medical Commission)
</TD></TR><TR><TD style="PADDING-BOTTOM: 0px; PADDING-LEFT: 8px; PADDING-RIGHT: 4px; PADDING-TOP: 8px" colSpan=4>The Panel meets twice a year in March and October.
David Butler is invited to the Panel meetings to represent all disabled applicants.
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The only minor glitch that I can foresee is that the review panel currently only sit twice a year (March and October) which is cutting it fine for Rnd 1 at cadwell, but far from impossible.
A driver with disabilities has to display some funky looking 'D' stickers on their competition vehicle

http://shop.msauk.org/product_details.asp?id=4317
The current MSA regulations as they stand today
http://www.bmsad.co.uk/regulations
There are limitations as to what class of International competative motorsports a driver with disabilities can participate in. I'll be looking into this in a lot more detail over the coming weeks and will update accordingly.