Tony Williams stood in for John Glascock for a short while, probably only for a few shows. The Madison Square Garden concert from 1978 has Williams playing with Barriemore Barlow still on drums. Barlow quit after Glascock died. I read somewhere that Anderson didn't like Williams' playing but I think it's great.
Actually Barlow, John Evan and David Palmer quit when Chrysalis leaked the news about a new Jethro Tull line-up that played on the "A" album that was supposed to be an Ian Anderson solo album.
Glascock died when the band was on tour supporting "Stormwatch" and the line-up (Anderson, Barre, Evan, Palmer, Barlow and Pegg) toured well into 1980.
Jeffrey Hammond-Hammond was great visually and played fine too but my favourites were John Glascock and Dave Pegg. Peggy's "jigs" and bass solos are great on "Stormwatch" and "A" tour bootlegs.
Tony Williams was a guitar player and played a fine bass. Much better than Anderson's over-amped-and-flanged bass on majority of "Stormwatch".