J. Leslie Martin, co-editor of 'Circle', and his wife Sadie Speight were enthusiasts for Jack's paintings of which they purchased several.  Leslie had been appointed Head of the Hull School of Architecture in 1934, at the age of only 26.  By 1937 Jack had arrived at the conclusion that architecture "offered a better chance of ever being able to earn my own living and seemed a logical progression from my painting." Leslie Martin offered him a place to train at Hull.  The Hull School of Architecture was relatively small but Martin managed to attract very high calibre visiting lecturers including Jacob Bronowski, László Moholy-Nagy, Marcel Breuer and Herbert Read.

Here is a reminiscence written by Jack shortly before his death as a contribution to A life so far… Recollections of Hull School of Architecture, edited by Jill Dickens, 2003:

From a letter to David Buckman, December 1993:
He later worked for a time in the office of the city architect in Coventry and between 1943 and 1946 served with the Royal Engineers in Britain, the Sudan (in Juba) and Palestine.

I was a student at Hull from sometime in 1937 (presumably from the start of the school year) until 1939 (presumably until the end of the school year).  Certainly until after the start of World War Two and I think after the birth of the Martins' baby daughter, Susan.  I have a memory of joining Leslie and Sadie (and baby?) under the stairs of their house in the Park (Pearson Park?) the night of the day war was declared expecting the bombs to start falling like rain.  Nothing happened.
Nothing happened for weeks or months.  By the time they did start I was away in Liverpool helping to build a war factory, and the Martins had moved to Boxmoor (near Watford), Leslie having been appointed deputy architect to the LMS Railway, a job he held until nationalisation after the war, except that he became chief architect at some time.
But my memories of the School of Architecture?  Very sketchy after 65 years.  You must treat anything I say with reserve.  The school was then a function of the local council.  It was one of Leslie's ambitions for it to be taken into the university.  I think this may have happened later.  Or didn't it?  Hence the present move? [A move of the School to Lincoln was proposed in 2003] I remember the school as being housed in the building of the school of art, an oldish gothic style building.  We had a large studio and a lecture room on the first floor.  Leslie gave lectures on the history of architecture, and no doubt other subjects.  Bernard Le Mare (assistant lecturer) on building construction.  I do not think Sadie gave lectures.  In the studio we each had our drawing board, Leslie and Sadie were both very beautiful draughtsmen, as a result of the old fashioned academic training they had had as students at Manchester?  They would go from board to board making encouraging comments on the work.  If you were lucky they would borrow your stool and pencil in order to touch up your drawing and demonstrate good lettering almost too perfect to be achieved.  If you were doing an interior with furniture Sadie would often touch in a little black stool or other amusing little additions perfectly drawn.  Leslie would sometimes sketch in freehand a faultless perspective.  If only one could achieve such merit!
By the outbreak of war I had obtained my [Royal Institute of British Architects (R.I.B.A.)] Intermediate and was working as an assistant in the Martin's office. I was painting as well, of course, strongly encouraged by the Martins. Before long it became necessary to find other, war related work, and, sometime in 1940, I joined Professor Holford's team building ordnance factories and various other war jobs in Lancashire and London. All this time, while living in various lodgings, I continued to work for my architecture finals. Although my various landladies were willing to accept a drawing board on their parlour tables they would probably have drawn the line at oil paints! In any case I was at work during the day and at night frequently patrolling the site wearing an LDV arm band and a tin hat and, armed with a 12-bore shotgun, hoping to pot the German parachutists as they landed.
From THE LONDON GAZETTE, 14 March 1941

NOTICE is hereby given that ARTHUR JACKSON HEPWORTH of Box Lane House, Boxmoor in the county of Hertford, lately called Arthur Hepworth Jackson has assumed and intends henceforth upon all occasions and at all times to sign and use and to be called and known by the name of Arthur Jackson Hepworth in lieu of and substitution for his former name of Arthur Hepworth Jackson and that such change of name is formally declared and evidenced by a deed under his hand-and seal dated the 10th day of December 1940 duly executed and attested and enrolled in the Central Office of the Supreme Court of Judicature on the 26th day of February, 1941.
Dated this fifth day of March, 1941.
BAILY and WILLIAMS, of King Street, Saffron Walden, Solicitors for the said Arthur Jackson Hepworth.
Box Lane House, Boxmoor was presumably the home of Leslie & Sadie Martin