I'm sure we've had this conversation before.
"tad" here is used as a sort of reinforcer, so the whole phrase means something like "yes, certainly!".
If you are able to re-watch tonight's episode (e.g. on Clic, or the S4C phone app), you'll hear "yndy tad" used at least once. (That's actually why I used it - I thought you might recognise it. It's quite common on RaR.
I was puzzled by its derivation for a long time, but eventually found out, somewhere online, that it probably was a reference to the almighty (as many "oaths" are, in English as well as Welsh).
So it might once have had the idea of "yes, by god!", or something similar.