Not sure if either of you have noticed but the marriage entry of 1803 is written as:
Robert Rogers Thomas of the parish of Llansilin.
Two points arise from this ~
* It was possible that it was either an error or a patronymic form might have been used initially - maybe son of Roger Thomas
* he was 'of the parish of Llansilin' at his marriage, not necessarily born there. I have a few ancestors from that area and found that I had to consult quite a few of the nearby parishes (in Montgomeryshire, Merionethshire as well as Denbighsire and Shropshire) to be certain of the correct births/marraiges.
Gadget
Yes, thanks, for this. I did notice the correction in the 1803 marriage but (a) Robert Thomas was certainly my gtgrandmother Catharine's father. You may be correct about it being a patronymic form. As I am sure we all know, in this line of work and for this and earlier periods of our family histories we are dealing with probabilities rather than certainties. I am reasonably confident that I have the correct baptism. The redacted "Rogers" (not Roger) in the 1803 marriage at Oswestry looks like a correction, and I will accept it as such. But, thanks, for the cautionary note. On your second point, yes, I could extend the search to the surrounding counties but given that Robert Thomas is a pretty common name the likely consequence would be to mushroom probabilities without increasing certainty.
Just seen Gadget's update pointing to Thomas Thomas as witness, likely brother, which I had also seen.
Thanks also to Mar again. We seem to be on the same page.
Many thanks to all
Best
John