Try contacting the War Museum as well - they were very helpful when I wanted to find my Dad's war records. You can go & search in person (you have to prove you are related to that person). Also, even if you haven't got your father-in-law's service number, you could try contacing his regiment (if they're not defunct now). For example, the Paras have their own museum, as do a lot of the old regiments and at least that would be a starting point - they could advise you where to go next.