Hmmmmnn, difficult one, it rather depends on how savvy the HR dept is.

Ultimately they would have to make a case for dismissal on the grounds of "capacity to carry out the required duties". This would require medical reports, and they would have to be able to evidence that they had made every effort to enable you to carry on working.

The words to remember are "reasonable adjustments" - under the disability discrimination act they have to make reasonable adjustments to accomodate your disability. It would be a reasonable adjustment to return to the previous method of working that you were able to cope with.

Make sure that you have a copy of the greivance policy and procedure just in case you need it.

If you are not a member of a trade union it could be worth considering joining one as they can provide support if your employer is unreasonable.

Hope all goes well for you, I do understand what you mean about it being easier to manage of couple of long days rather than more shorter ones - that is the pattern that suits me too.