[posted to both utl and uk.railway]

This is effective as of Sunday 12 September - at present the Sunday
service is half-hourly. The Monday-Saturday service will remain half-
hourly.

https://www.heathrowconnect.com/content.asp?pageid=73

Seems a rather retrograde move.

Looking at FGW timetable booklet number 8, it appears this is nothing
new and part of the plan all along:
http://www.firstgreatwestern.co.uk/Content.aspx?id=4934

This will also lead to a reduction in local services between Hayes &
Harlington, Southall, Ealing Broadway and Paddington from 4tph to 3tph
(Hanwell and West Ealing aren't served at all on Sundays) - i.e. down
to one HC and two FGW Reading locals per hour.

That said, these services were/are a bit bunched together rather than
spread out over the hour, so perhaps it sounds a bit worse than it is
in terms of local journeys - but the reduction down to one train per
hour to Heathrow won't do much to encourage local people on the line
to use the rail option to get to the airport, and it's hardly good
news for airport workers who currently rely on the service.