In reply to purplemonkeyelephant:
short and honest is the best answer. "certificate not available", don't embellish with speculation as to whether there ever was one or not. It won't look dodgy, I would a low percentage of householders can produce paperwork on demand from 5 years ago never mind 15 and solicitors are absolutely used to it.
Has it recently been serviced? If so the service record would be useful to provide instead as my recollection is that with gas appliances, "last person that touched it owns it" from a compliance/safety point of view