I'm sorry that I don't know the number during the era of the Tyneside electrics, though I can confirm that you are absolutely correct that at least 3 east-facing bay platforms have gone. Similarly, the west facing bays included platforms north west of the main roofline - within what is now the staff car-park.
Both Bradford stations had many more in days gone by, with just 7 spread between the two of them now.
Fenchurch Street I think had more than today's four platforms, on its Liverpool Street side.
Just 7 and 8 gained at Newcastle with electrification. They punched a couple of doorways in the old station back, and replaced one of the old running lines with a new double-length platform which serves what was the next running line back again.
Not quite, P5 and P6 were also new at the same time as P7 and P8. Pre-electrification that whole island didn't exist, there was just the back wall of the station, but with a few tracks (maybe 5 or 6) between the original P10 and the back wall - these were mainly used for carriage stabling. The footbridge across to the new island was newly built at the time, although successfully designed to look much older.
You're quite right about the gaps being knocked in the wall, but the pre electrification layout only had the three through platforms, 8, 9 and 10, these are now 2, 3 and 4. It was a major constraint on capacity - the new island with its split platform numbering was planned in conjunction with the closure of the line along the north bank of the Tyne through Scotswood to allow for through trains on the Sunderland - Carlisle (via Metro Centre) route.
I am pretty sure Fenchurch Street has never had more than four platforms
Blackburn had 8 (I think) until resignalling when Preston PSB was commissioned back in the early 1970s;
Hull lost a lot of platforms back in the 1980s and retained its northernmost bays only;
Those of us over a certain age can undoubtedly remember many, many dozens of stations that used to have more platforms than they do today (I clearly remember four platforms and significant station buildings at Blackpool South, for example) and also many large stations of which there is now absolutely no trace whatsoever.I have to say, it's disheartening to hear about all these losses at first, but then you do soon realise that if most of these were open today (even if all the closed branches were still in use!) that many of them would just never get used what with today's technology.
Malton now has one, but it used to have 3: a South facing bay (to Pickering I believe) and 2 through platforms - the York-bound line having a platform on either side.
I'm surprised Crewe hasn't come up yet. Platform 12 is the last remaining platform in use on the island built later than the rest of the station, which used to have two North and two South facing bays, plus a second through platform on the opposite side of the island.
Well, Hull still has 8 (IIRC, don't quote me on that, I can't remember if they took 8 out when they did the new interchange) but there are what appears to be 3-4 bay platforms to the south of Platform 1 which I have seen from time to time to keep stock temporarily... I've heard from my parents (so again... don't quote me on this) that those platforms to the south were mainly used for the Hull-Liverpool Im/Emigration link
If I recall, the Eureka building was the original main entrance, then some more platform lines were built so it was "marooned" on an island platform. These tracks have since been removed but the later bridge access is still used as the entrance to the station.
Manchester area - Fairfield
I'm not 100% sure about this but does Stratford still have a disued platform from when the DLR transfered from a single platform shared between DLR and National Rail to its current 4a & 4b site?