We have breeding Mandarins and good numbers of them in East Somerset. 30 together during January.
I have also seen them in West Wiltshire, just over the County border.
There are breeding mandarin at Yeovil, Dartmoor and forest of Dean, none of which are too far, plus free flying birds on a pond in north Somerset
I was a bit surprised to see a drake Mandarin on the wet area off Tealham Moor Drove this afternoon, feeding with the assembles Canada & Greylag Geese and Mute Swans. Had it flown in or wandered from a nearby wildfowl collection I wonder. Singing Sedge Warbler was the only other bird of note.