The Australian hybrid has also been seen a few times at steart. Plus S.African / Cape in 2021:
https://somersetbirding.org.uk/forum/viewthread/2728/

Interestingly I saw an Australian Shelduck at West moor, among up to 53 shelduck, on three visits between 14th and 24th Dec.  The moor was completely flooded on these visits and was packed with waterfowl, with maima of 1000 Pintail, 6540 wigeon, 1200 teal, 210 shoveler and 87 tufted ducks. Other highlights were 2 scaup, a single goldeneye, 1 ruff and 180 dunlin. It’s an amazing site when it’s flooded and during the post flood drawdown period.

I had a Cape Shelduck in Sand Bay in late November and I know it had been seen by Marc Hunt at Steart a couple of weeks prior to that.
Howard Taffs had the three Ruddy Shelducks over Brean Down in mid-October I believe so they have both been in the area quite a while.
Main problem with feral birds is hybridization I suppose but they do make a welcome addition to patch lists!

Escapes or from the Dutch feral population. 3 Ruddys did tour Somerset and Devon earlier in the autumn, although reduced to 2 when they came around the second time!

I think they might hang about it as they were with the Egyptian geese.  Of course, as the moor dries out many of the birds will move on. 

(Bear in mind that these are almost certainly escapes and thus in category D and not strictly tickable.)

Did the Ruddys settle or just flyby?

With the floodwater receding rapidly, the moor has been inundated with birds: 3000+ ducks mainly teal and wigeon with 500 pintail and 200 shoveler, two thousand lapwing, 25 black-tailed godwits, 110 cattle egrets, 4 great egrets, peregrine, marsh harrier, sparrowhawk, 16 Egyptian geese, 3 ruddy shelduck (one showing features typical of Cape shelduck) and 33 common shelduck.

Bluesky post showing some photos: https://bsky.app/profile/parliamentarygoose.bsky.social/post/3lek3q4j7ik2h.