Wrong signature

I have iPhone 4S and ipad mini 64gb. I use both for my private and work emails. I have separate signatures set up for each account. My msn account is first and work account is second. Both are through exchange sync.

I use both accounts many times a day but at least once a week the work signature is mysteriously replaced by my msn one (they then match)

This is really annoying as I end up giving my personal details to customers if I don't realise before sending.

I can not find any coincidences ie it is not after updates or iCloud back ups it is quite random.

Any ideas???