There have been two pairs at Abberton over the last couple of years. The first pair commuted between Abberton and Heybridge and were both ringed (red over yellow if I remember correctly but don't quote me on that). This year's pair were unringed and the consensus was that they were different birds as it was thought unlikely that both had managed to lose all their rings.
Wasnt aware of a second pair at Abberton...one of the pair I,ve seen there,as you say,had red and yellow rings,I feel that the Mersea bird is probably genuine,recent weather conditions and time of year being good for one of the small amount of RB geese that sometimes end up in western Europe getting caught up with the Brents,but almost certainly wont get accepted...
PH