Thanks to "Vinegar Vera" for sending me the result of his researches into this, which I have put at the bottom of the individual pages up to episode (130). It demonstrated that it might be possible to use listings data in national newspapers even though particular episodes are not usually identifiable.
I'm going to download as much listings information as I can from the Guardian's archive, although as this is not accessible without paying a fee and it is quite tedious to use, so it might have to be done selectively. However, there are various reasons why listings data on its own can't be relied on and I'm appealing for people who still have Prisoner recordings on videocassette to do some cross checking. I'll try and summarise the listings data in table format so anyone can refer to it to find the estimated date for an episode in a particular region and see if it checks.
Here are some ways recordings can be used:
(87) on Granada. Guardian TV listings for 03/11/88 has the episode description "Erica is forced to tell Lizzie about the police investigation". (Estimate from Times has (88) for this date)
(109) on London. Guardian TV listings for 12/10/89 has the description "Caroline receives an obscene letter but is convinced that Michael will not do any harm". The listing for 02/11/89 is "Caroline turns Michael in to the police" which confirms this date as (112).
(118) on London. Guardian TV listings has the episode description "A fearful Doreen decides to sell her house".
(171) on London. Guardian TV listings has the episode description "A TV show campaigns for Lizzie's release".
(180) on London. Guardian TV listings has the episode description "Margo adjusts to the outside world".
(182) on London. Guardian TV listings has the episode description "The argument between Judy and Bea gets out of control".
(259) on London. Guardian TV listings for 05/10/91 has the episode description "Meg makes her feelings clear to Ian".
(287) on London. Thanks to whoever sent me a tape of Joan Ferguson's first two episodes. The news bulletin before (287) has an item about Princess Diana opening an extension to City Airport which dates the episode as 5th March 1992.
(317) on London. Guardian TV listings for 14/07/92 has the episode description "The inmates prepare for concert auditions". And then adds sarcastically "oh goodie".
(336) on London. Guardian TV listings for 29/09/92 has the episode description "Hazel is held at Wentworth to be extradiced for armed robbery". The description for 24/09/92 is "Margo suspects that Nola has been inside before, which fits with (335)
(347) on Granada. The following programme on tape "Tales of Glamour and Excess. Danny Sugarman on his encounters with Jim Morrison, The Doors and Iggy Pop" appears in Guardian TV listings for 30/09/91.
(354) on Granada. 27/10/91 dated from ???
(369) on Granada. Dated from onscreen TV listings for following day including "Jack The Ripper" at 9:00 and 10:40, which was listed for 14/01/92.
(375) on Granada. During a promo spot for the following day's "this morning", Judy Finnegan says "Ken Hom will be with us to celebrate Chinese New Year", which fell on the 4th February in 1992.
Unusually, the Guardian TV listings for 13/08/92 have brief episode descriptions for some of the regional listings. Thus we have "Reb is caught" for Anglia (437)? and "Mo is the victim of a stupid law" for Granada (431), but nothing for HTV, TSW or TVS.
(534) on Carlton. Message in PCBH Digest 7 from Andrew Humphrey "on Carlton TV (London) on Monday 4th, we had the episode where they use (Pixie's song) over the closing creidts instead of the usual song."
(564) on Granada. Guardian TV listings for 11/01/94 has episode description "Lou and Lexie are in cahoots and Willie's memory returns". For the 13/01/94 the episode listing is "Daphne becomes the newspaper's editor" (565)
(568) on Granada. Guardian TV listings for 25/01/94 has episode description "Nora receives advice from Nikki".
Message from Andrew Williamson in PCBH Digest 10 06/12/96 "I'll continue writing summaries as Scottish air them (last night they aired ep.688 - only 4 to go)"
(692) on Granada. Announcer over final credits says that a new series of "Married with children" starts next week at 23:10, and this appears instead of Prisoner in the Guardian TV listings for 16 Feb 1995.