Hi Hilary,
You may want to check out
http://www.memento-mori.co.uk/. They have indexes for Cumbernauld which may save time on looking for a gravestone, etc. However, this would depend if the Youngs were able to raise headstones at the time.
I would definitely have a look at the parish record of Robert and Janet's marriage if you haven't already. This would be the most likely record to give you some information. Unfortunately though some records are very plain and only mentioned the names of those being married; no other details.

Hopefully there is something there that you can start a search with.
I also have to say that, in my opinion, ScotlandsPeople is very reasonable with regards to their costs, especially when you consider the price of one certificate of England and Wales plus your postage and packaging. There are also lots of free index sites for Scotland too; FreeCen (for early censuses), FreeReg (only some counties are available here) to name but two.
hume
