There is a train from Haverfordwest leaving at 13.20, arriving 13.42. The 17.48 back arrives 18.11. The 11.50 T5 bus from Fishguard, calling at Letterston at 12.04, connects. The last T5 bus back from Haverfordwest Rail Station now leaves at 18.45, dropping off at Letterston  at 19.08 and Fishguard at 19.18.

Alternatively, or if there is an issue with the trains – worth checking https://tfw.wales/places/stations/haverfordwest on the day - the 12.10 322 bus from Haverfordwest Bus station arrives Whitland 12.48, and the 17.00 bus back arrives 17.45. The T5 at 10.50 from Fishguard and 11.04 from Letterston connects, as does the return bus at 18.45.