I don't know for sure but when you do eventually apply for a mortgage, the bank will want to see pay slips for the last 3 months to prove your level of income. If the salary is reduced as a result of being off sick then they will want an explanation and possibly evidence (eg a health insurance plan / mortgage protection policy) if you have any of those that supplements the income to demonstrate that your income remained unchanged. If your income has dropped then I'm guessing that the amount you could borrow (3 x joint income etc) would be based on the reduced income and not the pre-sickness income.
You also need to look into the cost of insurance to cover your mortgage / life insurance before you commit since if it is an ongoing problem, the cost of the premiums may well influence your decision of how much to borrow. My H has a history of cancer (I know this is completely different but just giving an example) - he simply cannot get critical illness / mortgage protection insurance now and so I had to know that as a last resort, I would be able to cover the mortgage payments by myself if he became ill again and lost his income.