In your circumstances and absent any other information, I'd make provision for each member of staff using their full allowance of sick pay at six months full pay which would mean a 50% addition to labour costs overall; managing this allowance (which is easily capable of being viewed by employees as an additional 'bank' to draw one and two day holidays from) is going to be very challenging from a transferred employee point of view and the only way I think I'd be able to approach it would be 'Bradford Factor' sticks and attendance bonus carrots. While I'm sure you know this already, it's also helpful to remember that holiday pay accrues during sick leave and, where required, is carried over into the next holiday year.
It seems like an excessively generous allowance which makes me wonder whether this is a public to private sector transfer (sometimes called a first generation transfer) and if that is the case, I'd also pay particular attention to the pension arrangements at the costing phase. As anyone who has ever carried out one of these transfers will be aware, the cost of providing an equivalent LGPS pension is extremely expensive.
Alternatively, and only you know whether this is the case, it could be a term inserted into the employment contract about three months ago to dissuade alternative tenders in which case it wouldn't be enforceable as a transfer term.
