Thank you for a prompt answer. It would seem that the evening half hour service that I thought may have started in '75 or so did not last very long.
I have now checked the timetables for 1975, 1976, 1977 & 1981. None of these contain a half hourly service into the evening. There were services approximately every 30 minutes in the morning & afternoon peak hours.
For example, from 1977 timetable:
Helsby dep. 06:36, 07:29, 07:56, 08:14, 08:51, hourly to 21:51 on saturdays,
or (SX) until 14:51, 16:14, 16:57, 17:39*, 18:11***, 18:51 then hourly to 21:51
plus a 16:37 Stanlow & Thornton to Rock Ferry
* - probably a misprint, but timetable shows this to Little Sutton only.
*** - To Hooton only
Rock Ferry dep. 06:19, 07:18, 07:28(SX), 07:48, 08:08(SX), 08:40, 09:38 then hourly to 15:38, 16:18(SX), 16:38 (SO), 16:58 (SX), 17:28(SX), 17:38(SO), 17:58(SX), 18:38 & hourly to 21:38, 22:38 (to Ellesmere Port)
plus Hooton to Helsby dep. 18:17(SX), 23:26
Things had gone downhill by the 1981 timetable. Still mostly hourly in daytime, but last trains were much earlier.
Helsby dep. 17:45 (as amended in supplement) to Rock Ferry,
plus 19:05(SX) to Hooton only
Rock Ferry to Helsby 17:49(SX) or 18:31(SO),
plus 18:40(SX) Hooton to Helsby
Going back a few years, you are partly correct in that the evening service had been improved in the mid 1970s, but only to hourly. This seems to have started in the May 1972 timetable. In the 1971 timetable, the last trains were
Helsby dep. 19:15 to Hooton
Hooton to Helsby dep 19:45
(Only a few peak hour trains ran throughout between Rock Ferry & Helsby; at other times there was an irregular shuttle between Hooton & Helsby)