I called the hot line number today to check for their advice.
There is a longish intro giving advice about the problems of parking in the area and suggesting that visitors should be there early in the week as it gets very busy at the weekends and to arrive at least 1 hour before dusk. Choose a fine day which is more likely to produce the aerial displays with a million birds in the air performing their dramatic formation flying.
It concludes by saying the birds are roosting at the RSPB Ham Wall reserve at present.
For information on the roosting starlings please phone the RSPB Avalon Marshes Starling Hotline - 07866 554142.
PS. It is not unusual for the birds to change their roosting sites from time to time.