IB is payable to people who are unable to work. You are allowed to earn up to £51 in "therapeutic work" and still receive IB. If you qualify for and receive IB and then later go back to work for a short period, then you can go back onto IB, non-means tested, within 52 weeks. From the 6th April, IB will be reduced by one pound for every pound you receive over £85 in private pension or other income.
SDA is payable to people who are severely disabled and do not receive a state pension or IB.
IB and SDA are both separate to Disability Living Allowance (DLA). DLA is not means tested and you can still claim DLA even if you still work.
You cannot claim both IB and SDA.
All three claims forms (esp. DLA) are long and tricky to fill in - the Citizens Advice Bureau will help you fill them in.
Ring the UK MS Society helpline on 0808 800 8000 for more information - they can also send you a disability benefits help pack by return.
Please don't miss this date. The important thing is to get the claim in in time. You can receive non-means tested IB after 6/4/2001 provided that you claimed prior to this date.