When Waterloo won the 1974 Eurovision Song Contest, ABBA were accused of plagiarizing a Tchaikovsky piano concerto and the song Build Me Up Buttercup. There is, in fact, a similarity between the chorus of Waterloo and the first part of the chorus of Build Me Up Buttercup. I don't know how to describe it, but I'm sure there is a musical term for it. The lyrics to the chorus of Waterloo would fit perfectly with the melody of that part of Build Me Up Buttercup.
I had never heard the claim before, regarding a Tchaikovsky concerto being plagiarised for an ABBA song. There are other claims where Wizzard's See My Baby Jive had a role in inspiring Benny and Björn to make Waterloo, but whichever claim is correct is up to them.
That Foundations song sounds rather a bit like Baby Love by the Supremes rather than Waterloo, phrasing wise - I suppose I may view it a bit differently because from what I'm picking up there's an I-III[?]-IV-V chord progression in C major for Foundations whereas ABBA's chorus is a rather straight forward I-IV in D major... switching to V-I-V, and then starting the refrain again, discounting the ascending quasi-bluesy diatonic bassline. The swung notes of "ti, la-la" may be a bit too simple to be merited as a copyrighted melody in my opinion because it's not nearly as complex enough or creative.
It's no surprise that ABBA's early music tends to sound like a deliberate review of what 1960s music had, and I also read Stikkan Anderson tend to buy songs in order to make Swedish hits and make a name for himself and Polar Music before ABBA was formed. (Made in Sweden: Studies in Popular Music - Routledge)
...There is, in fact, a similarity between the chorus of Waterloo and the first part of the chorus of Build Me Up Buttercup. I don't know how to describe it, but I'm sure there is a musical term for it...
When they sing 'Build - me up', the two notes (fourth resolving to third) harmonise with the two notes (sixth resolving to fifth) of 'Wa - terloo'. If you listen to the Waterloo chorus and try to block out the lead melody line, you can hear the Build Me Up melody snippet (sung as Wa - terloo of course!) as the harmony below the lead. This is all very routine though, nowhere near plagiarism. One musical similarity between the two, also exhibited in See My Baby Jive, is the repeated V after 'ever more' in Waterloo, which appears at both start and end of the Buttercup chorus, and after the three wo-ohs in the chorus of See My Baby Jive. To my ears, Waterloo owes much more to Wizzard than the Temptations Foundations. See My Baby Jive was pop perfection.
Although Waterloo's chorus is unashamedly bog-standard, the verse is relatively interesting because it makes extensive use of inverted chords. So after 'My my, at...' the chord changes from D to E7 but the bass carries on playing D. Then on '..poleon did', the chords are A and G but the bass descends through C# and B. It may not seem to make much difference but it does ratchet the catchiness up a notch.
On this bass cover you can hear the chord transition from 'My my' to '[at] Waterloo' and see the bass line stick doggedly to the same D note throughout, then descend with those two further inversions