Hi Sarah,
It's important to note that the concept of "significant interruption" in relation to the 4-year rule is very different from the concept of "abandonment".
For example, suppose you have an agricultural barn that you start using as a house. If you live in it for a continuous period of 4 years, without significant interruption, then it will become lawful as a house (this is assuming that you're not caught by the relatively new provisions relating to concealment). For this purpose, a significant interruption could be as little as a few months. For example, if you used the barn as a house for 2 years, then left it empty for 18 months, then started using it as a house again, then the 4-year rule would almost certainly be interrupted and would start again from the latter period.
However, once a property becomes lawful for a particular use, then it becomes much harder for this use to be abandoned. There are a number of factors that are used to assess "abandonment", including the period for which the property has been empty, but generally the property would have to be empty for many years before the use would be abandoned. With the above example, if you used the barn as a house for a continuous period 4 years, so that it became lawful as a house, and then left it empty for 18 months, then its use as a house would almost certainly NOT be abandoned.
In my opinion, from the situation that you've described it sounds like there would be a decent chance of an LDC being issued, although these types of issues can be very complicated so you may want to seek further advice from someone else (e.g. a legal professional, etc). I'm guessing that the reason the Council are asking for the death certificates is because if it turns out that the ladies died (say) 20 years ago, then there could be an issue of abandonment. However, if the period is only 18 months, and assuming that the property is still in a liveable condition and hasn't been used as something else during this period, then I would be surprised if this is considered to be abandonment.
Thanks,
Steve