Dear Lu, Janette and KHP,
I am a little overwhelmed by such a helpful and detailed response! I will let you know what the Probate records contain, and whether they show a relationship between David Allan (d. 1977) and David Allan (d. 2003).
I must also apologise for my original post breaching the rules insofar as it referenced the names of possible living relatives. I will bear this in mind with future posts.
Thank you all again!
Dfwdavies