I consulted the
1970 SCCU Grading List and found there were two chess-playing Whelans in 1970...
J A J Whelan - graded 177 (and 188 the previous year) of Sussex University
P J Whelan - graded 158 (and 157 the previous year) of Lloyds (Insurance League)
... who must be the people that John C and Roger de C are referring to.
Either J T Whelan is a third player or else he is a misspelling of one of the other two. I note that google searches for '"j a whelan" chess' yielded some references to matches recorded in SCCU bulletins but not so '"j t whelan" chess' which is perhaps an indicator of a misspelling.
People with three forename initials tend to be easy to find via FindMyPast so I looked up J A J Whelan. He was John Anthony Joseph Whelan, born 30 July 1945 (Amersham, Bucks), died 21 December 1981 (Kensington). The death also appeared in the probate record where he is referred to as John Anthony Joseph Whelan of Christ's College, Cambridge. Said college awards annual Dr J A J Whelan prizes. He was an astronomer and an obituary appeared in the
journal of the Royal Astronomical Society though it didn't mention chess.
Royal Astronomical Society journal wrote:JOHN WHELAN
Fourteen years ago John Whelan began studying for his masters degree in astrophysics at the University of Sussex. It was the start of an outstanding career of original research and teaching in astronomy which was exceptional for the range of subjects he contributed to, and the personal inspiration he provided for others. His death at the age of 36 after a long battle with cancer, fought with those qualities of determination, optimism and courage he showed throughout his life, is a loss to all who knew him, to the astronomical community to which he contributed so fully, and to the subject to which he was devoted.
John Whelan was born 1945 July 30, the eldest son of Mr and Mrs James Whelan. He gained a first-class degree in mathematics at King’s College, London, in 1967, before starting to study at the Astronomy Centre in Sussex. There he completed an MSc degree with distinction, followed by his DPhil research on contact binary stars, and the structure of rotating stars. The Astronomy Centre, and indeed the University of Sussex, was then in its early years, and much depended on the successes of its first students. From the beginning John Whelan applied himself to research with infectious enthusiasm and an outgoing spirit.
... He went to Cambridge in 1973 where he was a dedicated teacher of mathematics at Christ’s College, while at the same time giving himself fully to the supervision and advice of graduate students, and to research and administrative duties at the Institute of Astronomy. In 1975 he took a major part in organizing the IAU Colloquium at Cambridge on the ‘Structure and Evolution of Close Binary Stars’ and then spent a year as Staff Astronomer at the Anglo-Australian Observatory. ...
followed by a lot of technical astronomical data which can be
read online if anyone is interested. It doesn't mention chess anywhere.