Thanks for that, yes I remember the 1984 Manchester and Liverpool HST services, as discussed in another thread. Interesting to hear they come and go.
Interesting that there were a lot of changes in 1987 as that's the first year I don't really recall. That said, the 1986 pattern was significantly different to 1984, e.g IIRC it was as follows:
1984 northbound Reading approx times, accurate in some cases; 1985 largely unchanged
0830 Poole-Manchester
0941 Poole-Glasgow/Edinburgh Wessex Scot
1045 Brighton-Manchester
1137 Poole-Newcastle
1337 Poole-Manchester
1440 Brighton-Manchester
1545 Portsmouth-York
1637 Poole-Liverpool
1986:
0830 Poole-Manchester/Liverpool (?) (unchanged)
0940 Portsmouth-Liverpool (the famous 1M05 reintroduced and diverted to Liverpool after 2 years absence except Summer Saturdays)
1035 Poole-Glasgow/Edinburgh Wessex Scot (delayed 1 hour)
1140 untimetabled Reading-Birmingham which (IIRC) I observed at Reading on two separate occasions, Tyseley DMU, the nature of this service has been a long-standing mystery even these forums have been unable to solve

1236 Poole-Newcastle (delayed 1 hour)
1349 Paddington-Liverpool (didn't exist in 1984)
1434 Portsmouth-Manchester (didn't exist in 1984, the best ever year under BR for Portsmouth, 2 northbounds per day)
1635 Poole-Liverpool (unchanged)
The big 1986 changes were, it appears, mostly as a result of diverting the Brightons up the WCML with loco change at Willesden (?)
Getting further into detail on XC history (1979-1986), from my piecing together of XC timetables from various sources (marshalling books on the BR Coaching Stock groups.io group, old timetables on Timetable World, own memory, and this forum!), certain eras stand out on the Reading axis:
1979-81: relatively few changes during these three years. Close to perfect clockface 2-hourly New St-Manchester (odd hour xx55) and Liverpool (even hour xx55). Additional Manchesters on top of that (around xx10 odd hours). Brightons introduced. Some Paddington-Birmingham services extended to the northwest. Mostly clockface xx50 out of Paddington, mostly alternating between Birmingham (and beyond) and Worcester. Additional Poole-Manchester. Most Birmingham-NW services, however, to and from the SW and S Wales.
1982: significant changes, perhaps a knock-on from the withdrawal of direct Paddington-Worcester services with more Paddington-Birminghams, still mostly xx50 out of Paddington but not the same hours as the preceding years. Best ever Reading-Birmingham service from 1979-86 (slightly better than hourly) with a mix of Paddingtons, Pooles, Brightons and the brand new 1105 Portsmouth-Manchester. Poole-Newcastle moved two hours earlier. Birmingham-NW frequency largely maintained with some alterations and some loss of perfect clockface. Full timetable available on Timetable World.
1983: more significant changes. introduction of clockface xx37 Reading-New Street most hours, but now with a gap at 1237. Some significant Birmingham-Manchester reductions, with some two-hour gaps. Long gap between the 1300 and 1610 contrasts with a basically hourly service for the previous 4 years. Reduction in Paddington XC services, with just 4 per day (0630, 0730, 1337 and 2018 from Reading). The famous 1M05 now forms the Portsmouth-Manchester (departing 0805). Brighton and Poole times unchanged.
1984: more significant changes, again. Loss of clockface from New Street, which has been discussed in a recent thread and was apparently down to difficulties maintaining tight connections at New Street. Paddingtons further reduced, with just the two morning services. 1M05 now becomes an 0805 Portsmouth-Poole via Reading. Introduction of Portsmouth-York afternoon service (and southbound morning service) and southbound Liverpool-Portsmouth, so Portsmouth gets two southbound XCs for the first time. Introduction of the famous "Wessex Scot", which would last right up to Operation Princess.
1985: little change, but a number of services become diesel-hauled throughout, without the loco change at New Street. I wonder if this was related to transfer of some 86s to Liverpool St-Norwich?
1986: significant changes again (see above) driven largely by Brighton diversions up the WCML, I believe.
So yes, very frequent changes year-on-year during this period, with only the 1979 and 1984 timetables lasting basically more than a year (with tweaks).
If anyone can provide similar summaries for the few years or so after that without too much effort, it would be interesting to see!
A fair amount of stability was, however, provided during the immediate pre-Virgin and early Virgin years, with tweaks and additions but with the basic structure of services through Reading, including stock (47-hauled or HST, loco change at New Street or not) largely unchanged year on year.