I would send both back together in the same envelope (take copies, get proof of posting from PO) with a grovelling letter; unfamiliar with area, not driven in Plymouth for a few years, road used to be 30mph, etc. In this mention that due to the enforcement being via unmanned equipment, and the journeys just one day apart, you did not have chance to modify your driving as you would have had you been stopped. I mean obviously not seeing the terminal signs, repeaters, 20mph marking on the roads is a major contributing factor here and they may just bin it after a chuckle and a mention of cheeky bar stewards, but if you don't ask you don't get.
It is highly unlikely they will treat the two as a single offence, but it has been known for the Police to cancel a ticket where multiple offences of 3 or 4 instances have been recorded over a very short space of time. They may take pity even though it is just two offences.