Clubman, you are correct, the couple will remain on single assessment for the remainder of the year, then they will be reassessed and given any refund for 3/12 of the year in which they were married. This will be returned to each spouse in the % of the overpayment they have made i.e. if the husband is on higher tax he will get a higher repayment at year end.
Regarding the medical expenses, there is a higher cut off for a family claim, however it would normally be more beneficial to put in one claim than two seperate claims.