but your job is guaranteed - it's not like you're in the private sector and taking a career break... if you already have been paying a mortgage and are not borrowing excessively, you've nothing to lose..
Ask your employer to include a statement in the letter that you can return to work sooner than 5 years (cut the career break short) if finances demand....
see if your union has a mortage deal with certain banks - EBS has a deal with a union whereby they take members current salary and 3 increments into account when assessing members borrowing power...if you don't ask, you'll never know!
good luck!